Do you want to block people from adding you as a contact, or are you trying to prevent people on your network from using the add contacts feature?
If its the former:
You can fix this by changing your .netpassport/Hotmail profile.
- Sign on to Hotmail, go to Options, Personal Profile, uncheck all Share Information boxes and click Update.
- Next go to Member Directory and make sure the "Please list me in the HotmailMember Directory" box is unchecked and click OK (or Cancel if it is unchecked).
As far as I know, there isn't a way to prevent people on your network from adding contacts to their list. You could remove their access to MSN messenger entirely, and have them all use a chat program written in VB or C++ -- just use Winsock, its not too hard to code it if you've ever done anything with visual studio.