How to not let users remove the entity reference once they added

Question

I'm using D7.
I have one content type, let's say Content-A. And, the Content-A has a field whose field type is Entity Reference. Content-A's authors can reference other user with the content through the Entity Reference field.
I want to make authors be unable to remove the entity reference between the content and other users who he already referenced. Does anybody know how to do it? Any advice will be appreciated.

Answer 1

Using Hook_form_FORM_ID_alter on the node form and disabling the field for users that are not the initial author should work.

Code is untested but shows the general idea behind the solution

HOOK_form_FORM_ID_alter($form, &$form_state){
  global $user; // Get the current logged in user information
  if($user->uid != $form_state['node']->uid){ // Is the current user not the original author of this node? 
    $form['FIELD_REFERENCE']['und'][0]['#access'] = 0; // Restrict access to this field (You'll have to check where exactly in the tree to place this).
  }
}

Answer 2

You could use the field permissions module for this, give access to add but not edit their value. https://www.drupal.org/project/field_permissions