You can achieve using following hook_form_alter() steps:
1. In the hook_form_alter() function get the entities in which logged in user is referenced, this can be achieved by using the the view which i shown below. Get the query of the view and check whether logged in user is referenced in any band content type or not.
2. If any user is referenced in any node of Band content type then don't display node form and return directly
3. If user is not referenced then display content creation form of Band node type
In the following view i have kept field_referenced_users field which contains the referenced users in nodes. So change your field accordingly. Just import the below view and get the query, use this query in hook_form_alter() and check it. I have created view for article node type. You can change it accordingly.
$handler = $view->new_display('default', 'Master', 'default');
$handler->display->display_options['title'] = 'Entities in Which Logged in User is Referenced';
$handler->display->display_options['use_more_always'] = FALSE;
$handler->display->display_options['access']['type'] = 'perm';
$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'] = 'full';
$handler->display->display_options['pager']['options']['items_per_page'] = '10';
$handler->display->display_options['style_plugin'] = 'default';
$handler->display->display_options['row_plugin'] = 'node';
/* Sort criterion: Content: Post date */
$handler->display->display_options['sorts']['created']['id'] = 'created';
$handler->display->display_options['sorts']['created']['table'] = 'node';
$handler->display->display_options['sorts']['created']['field'] = 'created';
$handler->display->display_options['sorts']['created']['order'] = 'DESC';
/* Contextual filter: Content: Referenced users (field_referenced_users) */
$handler->display->display_options['arguments']['field_referenced_users_target_id']['id'] = 'field_referenced_users_target_id';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['table'] = 'field_data_field_referenced_users';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['field'] = 'field_referenced_users_target_id';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['default_action'] = 'default';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['default_argument_type'] = 'current_user';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['summary']['number_of_records'] = '0';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['summary']['format'] = 'default_summary';
$handler->display->display_options['arguments']['field_referenced_users_target_id']['summary_options']['items_per_page'] = '25';
/* Filter criterion: Content: Type */
$handler->display->display_options['filters']['type']['id'] = 'type';
$handler->display->display_options['filters']['type']['table'] = 'node';
$handler->display->display_options['filters']['type']['field'] = 'type';
$handler->display->display_options['filters']['type']['value'] = array(
'article' => 'article',
'page' => 'page',
);
$handler->display->display_options['filters']['type']['group'] = 1;
/* Display: Page */
$handler = $view->new_display('page', 'Page', 'page');
$handler->display->display_options['defaults']['hide_admin_links'] = FALSE;
$handler->display->display_options['path'] = 'users-referenced-entity';