This is a much more complex issue than just IP banning, since a great deal of the bot-spam that goes out is generated by compromised machines...like the one owned by Joe Dipshit down the block, who thinks a firewall is something he builds around the trash he burns in his backyard every Saturday, you know, to keep his double-wide from burning down.

There are entire organizations devoted to compiling RBLs (realtime blackhole lists) that many hosts (including the one you're on) use to cut down on spam email, site spam, etc..., but getting the spambags banned from the Interneck altogether is not so simple. An IP# becomes compromised by a spambag because his ISP allows it to happen. If they're not monitoring their networks and, ergo, not seeing the huge outflow of traffic, then they're idiots. If they are monitoring, and just don't care, well, that's not really something you can do much about, other than blocking them completely. Sometimes the pressure of complaints from their legitimate clients is enough to make them clean up their acts...and that's where the RBLs *are* helpful.

It is the ISPs that should be held accountable, and are by those of us who drop entire IP ranges (and even entire country codes) into blacklists. But there will always be ISPs for whom money is the ruling code, so between them and the compromised machines, there will always be a way for spammers to smear their shit on your windshield (so to speak).