Select users with only one role
If I were you (and with my poor level), I would proceed in 2 steps:


Get all the users with the role that interests you. Maybe inspire yourself for the query here: Get all users with specific roles using EntityFieldQuery
Browse the query result by loading each user and check if $user-&gt;roles has another role than the one you are looking for.
Push the final results in another array.


I don't know if it is a good way to proceed or if there is another way to achieve this, but I think it could do the trick.