From http://discussions.apple.com/thread.jspa?threadID=2016206
The question was “How do you temporarily disable web access on a mac?”. The answer was:
ipfw add 2005 deny tcp from any to any 80 out xmit en0
Replace en0 with whatever network interface is active. Someone else asked how to undo it. I would have responded but the thread is closed. Here is what i would have said:
ipfw delete 2005