A core Scrum and agile philosophy is to support communication over contract negotiation. By putting in processes within tools such as TFS really goes against the spirit of Scrum and reeks of micro-management. I would not not recommend going down this path.
However, if you insist on doing this you can edit the work item template and change the rules on the field to.
<READONLY for="TFS role or Domain Group" />
Note it only supports TFS groups and domain groups and you cannot restrict it to a user.