Is it possible limit who a technician or group of technicians can assign a request to?

Our tiered support (1-4) reside in different support groups.  We're having issues with Tier-1 skipping and assigning to T-3.  We would like the only option to be for them to assign to T2 or to reassign to other groups that are not in the tiered group.

Additionally, we want to make sure that technicians are only assigning to groups and not individual technicians. Is that possible?

Thanks,

Adam