Great stuff and thank you for that early information, how to disable MAPI per user.
Unfortunaly the Exchange 2003 SP2 CTP does not expose that settings in the GUI.
And on the other side it's not really a fun to maintain hundreds or thousand of users manually. And i have not found a way to control access by group member ships. So i have written a small VBScript earlier to set the POP3, IMAP4, OWA, OMA-Permissions using security groups.
I have added the MAPI-Properties with the current version. So i have to create group to allow POP3, IMAP4, OMA, OWA and to disallow MAPI and to enforce "cached mode". and start that vbscript once a day.
Unfortunaly the web page itself is german.So use google to translate. But VBscript is "universal" :-)
http://www.msxfaq.de/tools/grp2exinet.htm