How can I restrict permissions to change state to Closed?

How can I restrict permissions on work items so that only certain people have the right to close them? Better yet, how can I further filter this restriction to limit this right based upon thetype of work item?

Here are two examples:

1. I would like to add a restriction that only members of the QA group can change the state of abug-type work item to closed.
2. I would like to add a restriction that only members of the Product Management group can change the state of aquality of service-type work item to closed.

[633 byte] By [Dave_Novak] at [2008-2-10]