'Blackholing' certain domains

[Jason]

Hello everyone.

I just put up a BIND 9 server for our company intranet. Everything is 
working great (Better than expected really :) ).

Anyway, I want to block certain domains so my users can't get to certain 
services, specifically IM stuff.

I was curious what the best way is to go about doing this. Obviously, 
you need to create a zone file, but what should the contents of the zone 
file be?

Anything I should put in the zone file to block it? Maybe leave it blank?

I appreciate it.

Jason