I don't disagree with that, but please do some investigation before banning an entire subnet, or wait until you actually observe a user jumping around the subnet. And never ban an entire ISP if you haven't first given their abuse department a chance to do the right thing.
If you don't do this, you're harming innocent people.
Your warning has merit, but it's a little out-of-scope for the problem. I'm not talking about banning people from playing Halo, I'm banning them from just my server for medium durations like a day or a week. And chances are they're the only person connecting to my server from that ISP -- each of the ~30 people on the server probably has a different ISP.
If you don't do this, you're harming innocent people.