Block email coming to Exchange 5.5 Srver

We have NT 4.0 domain running DNS services & Exchange 5.5 Server with SP4 & Mcafee Netwhield & Groupshield. I need to block & deny emails coming to us from “support@microsoft.com” & few others. Whats the best way & cost effective way. Are there any freeware tools ?
I checked on MailMarshal but it works only on a Win2000 Server running AD & Exchange 2000.
Akash