And what's to stop them from serving up a slightly different IP address every time?

A shared list of banned IP addresses is a bad idea. Dynamic IPs change regularly. You can argue the bad guys aren't likely to use dynamic IPs, but if they're smart enough to do all the other stuff that's alleged it certainly won't take them long to start changing IPs. Plus, a shared list is just begging for problems. Someone with a grudge will start adding IPs just to get back at people. And the bad guys could simply add the IPs of people sharing tons of files. Then we'd all end up banning the wrong people.