Prevent Worked on Items in Queue from Assinging to another User

Hi, I have looked through security roles, but see no where to manage security on Email activities in a queue created by the Email router.

I want users in a support team, that once they 'Work On' a queue item, they can only release it back to the queue. Currently they can assign 'Worked By' to anyone, which I want to prevent.

I tried setting Queue and Activity (Assing) security in the Security Roles, but this does not work.

Any Ideas?