How to stop Privatemsg showing user role in 'To' field
Inside the privatemsg module, in hook_menu search for $items['messages/autocomplete'] this is where the menu defines the auto complete path.

Search for privatemsg_autocomplete function where the output is defined.

UPDATE:

To remove just the role, here is a patch for that. Here is a thread on drupal that shows how this has progressed https://drupal.org/node/1852746