Normally this is done by "role", i.e. only someone with a particular
kind of rule can set/modify the owner of a work item in a given team
area. In particular, to control what roles can perform this action, see:
Team Configuration : Permissions : Permitted Actions
: Work Items : Save Work Item : Modify the work item's owner
To have a rule that says someone who is allowed to assign work cannot
assign work to themselves is not a rule I've ever heard being used, so
that isn't provided out-of-the-box ... for that you'd have to write your
own Save-Work-Item process extension (or reconsider the necessity of
this rule :-).

As a side comment, although it makes sense to have certain types of work
items which are tightly controlled, I strongly encourage every
organization to have some type of work item (perhaps a PersonalTask work
item type) that a developer is free to create and assign to themselves,
so that a developer can manage the details of their own work using Team
Concert. Otherwise, you use many of the benefits of the Agile Planning
tool, if a developer cannot add entries to accurately capture how they
are spending their time.

Cheers,
Geoff

donnamlamb wrote:

Is there a way that we could prevent a user from assigning a work item
to themselves. Maybe by preventing the current user from being listed
in the Owned By drop-down list? Any ideas that would not necessitate
creating a plug-in would be helpful. We have a "separation of
duties" rule that requires not only can someone not assign a
work item to themselves, but that the few people who ARE allowed to
modify the Owned By field are also not allowed to assign the WI to
themselves.