How to block Yahoo and MSN messenger using ISA Server 2004 Enterprise Edition?

How to block MSN and Yahoo messenger in a domain using ISA server 2004 Enterprise edition.? The way Microsoft describe to block i tried butt it didnt work.
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Ahmed Abdel SalamPh.D. CandidateCommented:
you can block it using the application header signiture
Even if you block it with the header signature clients can still access msn and yahoo messenger through their mail websites.  You will have to create a url block as well.

Check out this for additional info:
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