Andy --
This is one of only a few features in the Project Server 2007 that the
Project Server administrator CANNOT disable. The fact that you are asking
about it probably reveals that one of your PMs accidentally APPROVED a
delete assignment request.
So, because you cannot disable it, you need to make this feature a training
and performance issue with both project managers AND team members. Teach
your team members NOT to use the feature, teach your PMs to be alert to
delete assignment requests on the Task Updates page and to REJECT them if
they see any of them, and then do you best to hold them accountable for
their compliance to your training requirements. Hope this helps.