It's going to be a little more involved than that. You could do it this way (there are probably more, but this is the one that comes to mind immediately).
First off, you're going to have to be comfortable with editing your work item type definitions.
First off, install the TFS Power Tools 2010 or 2008. This will provide you with the tools you need to edit the work item type.
Once this is done, open visual studio, and go to the menu Tools->Process Editor->Open WIT from Server
Open the work item type you want to change the Assigned To drop down list. Double click on Assigned To
, go to Rules
and double click on ALLOWEDVALUES
.
When you do this, you'll see the list of allowed values. In our particular case, it is restricted to [project]\Contributors and [project]\Moderators
What you could do is create a new group in the TFS project (or at the server level) called Assignees (or something) with all of the people you want to be able to assign, then delete the allowed values that are in the work item, and add [project]\Assignees.
Once you are done, you can save the Work Item Type definition back to the server. You'll have to refresh (close and reopen visual studio, for instance) for the changes to be reflected correctly.
Alternatively, you might be able to create a Global List and use that as a list of exclusion values. I'd need to try this in a controlled environment first.