Blocking Dangerous Instant Messenger Applications
So many small (and otherwise) businesses have employees abusing Instant Messaging at work these days that many of us are being asked to block IM. If you'd like to know how to prevent these applications from functioning using ISA take a look at Tom Shinder's excellent article over at isaserver.org.
How to Block Dangerous Instant Messengers Using ISA Server
Note that in Tom's article he recommends changing the HTTP redirector settings to Reject HTTP Requests from Firewall and SecureNat Clients. This may not be the best thing for SBS users so be careful and test this recommendation if you decide to implement that portion. The information in the article prior to this point should be sufficient to thwart most IM applications.