Disabling PM's for specific users?
 

How do we disable the PM facility for specific users?

I understand that we can turn PM's off for Registered Users, create a second usergroup with PM facility, and add all users except this one to it.

However, as we get regular users signing up every day, we don't have the manpower to keep manually putting people in this secondary usergroup.
We can't use an auto promotion system, because as soon as we remove the one particular user from the secondary group, they'll get automatically updated again.

Ideally, I'd like something like the forum access masks....but for PM's?