As of Office XP/Outlook 2002 SP2 the DisablePST setting will totally
disable the creation of PST files. If you don't have SP2 applied then
it will not totally disable it.
Another alternative, although more drastic and troublesome is to edit
the MAPISVC.INF file and remove the reference to PST file support.
You'll have to play a bit to get it right. Be warned though, it
doesn't give pleasent error messages to users, but it will totally
stop PST file usage.
As a side issue - disabling PST files using the MAPISVC file will
prevent KVSVault from working, unless you set a certain set of options
on the client and server.
Michael