Why not just make sure they adhere to the rules of the forums, just as everyone else does. If they (donater) decides they are now all and mighty after forking a few bucks then refer them to the amended rules (place new amendent about donations). If the person is still having issues, ban them, and leave it at that.
After all, it's a donation and not a subscription to "RANT AND B!TCH WEEKLY."
Just my two cents...no wait, my $25.00 I donated at MW06.