Here is a Rules/Views heavy solution without any custom code.
Modules Used
- Views
- Views Bulk Operations
- Rules
- User Relationships
- User Relationship Rules
What To Do
The User Relationships module ships with an addon module called User Relationship Rules. This will need to be enabled along with the other modules listed above.
Using the Rules module, create a component that removes a pending user relationship. (See additional resources below.)
Next create a View that lists the pending User Relationships as you have done as noted in your comments.
Next you will need to add the Views Bulk Operations field to your view and configure it to use the rules component you created earlier.
Additional Resources
Johan Falk has several videos you may find useful in setting up your VBO view.
He has also put together a fantastic introduction to the Rules module.
Views Resources
Enable the bundled UR-Views module (User Relationship Views) and enable the view titled user_relationships_browser
at admin/structure/views
This enables an example view that has a relationship added for both the Requester and Requestee accounts. You should be able to use these to populate both user fields for your Rules component.

You should end up with a list of pending user relationships with checkboxes. Views Bulk Operations will run each selected item through the Rules component you created allowing an Administrator to delete pending user relationship requests between users.
Exported View - Retrieves Both Requester and Requestee UID's
$view = new view();
$view->name = 'user_relationships_browser';
$view->description = 'Shows someone\'s related users';
$view->tag = '';
$view->base_table = 'user_relationships';
$view->human_name = '';
$view->core = 0;
$view->api_version = '3.0';
$view->disabled = FALSE; /* Edit this to true to make a default view disabled initially */
/* Display: Defaults */
$handler = $view->new_display('default', 'Defaults', 'default');
$handler->display->display_options['title'] = 'Relationships';
$handler->display->display_options['use_more_always'] = FALSE;
$handler->display->display_options['access']['type'] = 'none';
$handler->display->display_options['cache']['type'] = 'none';
$handler->display->display_options['query']['type'] = 'views_query';
$handler->display->display_options['exposed_form']['type'] = 'basic';
$handler->display->display_options['pager']['type'] = 'none';
$handler->display->display_options['style_plugin'] = 'table';
$handler->display->display_options['style_options']['columns'] = array(
'plural_name' => 'plural_name',
'name' => 'name',
'changed' => 'changed',
);
$handler->display->display_options['style_options']['default'] = '-1';
$handler->display->display_options['style_options']['info'] = array(
'plural_name' => array(
'sortable' => 0,
'default_sort_order' => 'asc',
'align' => '',
'separator' => '',
'empty_column' => 0,
),
'name' => array(
'sortable' => 0,
'default_sort_order' => 'asc',
'align' => '',
'separator' => '',
'empty_column' => 0,
),
'changed' => array(
'sortable' => 0,
'default_sort_order' => 'asc',
'align' => '',
'separator' => '',
'empty_column' => 0,
),
);
/* Relationship: User relationships: Requestee user */
$handler->display->display_options['relationships']['requestee_id']['id'] = 'requestee_id';
$handler->display->display_options['relationships']['requestee_id']['table'] = 'user_relationships';
$handler->display->display_options['relationships']['requestee_id']['field'] = 'requestee_id';
/* Relationship: User relationships: Requester user */
$handler->display->display_options['relationships']['requester_id']['id'] = 'requester_id';
$handler->display->display_options['relationships']['requester_id']['table'] = 'user_relationships';
$handler->display->display_options['relationships']['requester_id']['field'] = 'requester_id';
/* Relationship: User relationships: RTID */
$handler->display->display_options['relationships']['rtid']['id'] = 'rtid';
$handler->display->display_options['relationships']['rtid']['table'] = 'user_relationships';
$handler->display->display_options['relationships']['rtid']['field'] = 'rtid';
/* Field: User: Uid */
$handler->display->display_options['fields']['uid']['id'] = 'uid';
$handler->display->display_options['fields']['uid']['table'] = 'users';
$handler->display->display_options['fields']['uid']['field'] = 'uid';
$handler->display->display_options['fields']['uid']['relationship'] = 'requestee_id';
$handler->display->display_options['fields']['uid']['label'] = 'Requestee User ID';
/* Field: User: Uid */
$handler->display->display_options['fields']['uid_1']['id'] = 'uid_1';
$handler->display->display_options['fields']['uid_1']['table'] = 'users';
$handler->display->display_options['fields']['uid_1']['field'] = 'uid';
$handler->display->display_options['fields']['uid_1']['relationship'] = 'requester_id';
$handler->display->display_options['fields']['uid_1']['label'] = 'Requester UID';
/* Sort criterion: User relationship types: Plural name */
$handler->display->display_options['sorts']['plural_name']['id'] = 'plural_name';
$handler->display->display_options['sorts']['plural_name']['table'] = 'user_relationship_types';
$handler->display->display_options['sorts']['plural_name']['field'] = 'plural_name';
$handler->display->display_options['sorts']['plural_name']['relationship'] = 'rtid';
/* Contextual filter: User relationships: Requester user */
$handler->display->display_options['arguments']['requester_id']['id'] = 'requester_id';
$handler->display->display_options['arguments']['requester_id']['table'] = 'user_relationships';
$handler->display->display_options['arguments']['requester_id']['field'] = 'requester_id';
$handler->display->display_options['arguments']['requester_id']['default_action'] = 'default';
$handler->display->display_options['arguments']['requester_id']['exception']['title_enable'] = TRUE;
$handler->display->display_options['arguments']['requester_id']['default_argument_type'] = 'user';
$handler->display->display_options['arguments']['requester_id']['default_argument_options']['user'] = FALSE;
$handler->display->display_options['arguments']['requester_id']['summary']['number_of_records'] = '0';
$handler->display->display_options['arguments']['requester_id']['summary']['format'] = 'default_summary';
$handler->display->display_options['arguments']['requester_id']['summary_options']['items_per_page'] = '25';
$handler->display->display_options['arguments']['requester_id']['specify_validation'] = TRUE;
$handler->display->display_options['arguments']['requester_id']['validate']['type'] = 'numeric';
/* Filter criterion: User relationships: Relationship status */
$handler->display->display_options['filters']['approved']['id'] = 'approved';
$handler->display->display_options['filters']['approved']['table'] = 'user_relationships';
$handler->display->display_options['filters']['approved']['field'] = 'approved';
$handler->display->display_options['filters']['approved']['value'] = '1';
$handler->display->display_options['filters']['approved']['group'] = '0';
$handler->display->display_options['filters']['approved']['expose']['operator'] = FALSE;
/* Display: Page */
$handler = $view->new_display('page', 'Page', 'page_1');
$handler->display->display_options['defaults']['hide_admin_links'] = FALSE;
$handler->display->display_options['path'] = 'admin/manage-relationships/%';
$handler->display->display_options['menu']['type'] = 'normal';
$handler->display->display_options['menu']['title'] = 'All Relationships';
$handler->display->display_options['menu']['weight'] = '0';