Why do ISP's forward spam messages?

If I type someone's email address even a teensy bit wrong, it bounces back. But I routinely deal with nasty, icky, spam (porn, near-porn, etc.) messages that aren't even addressed to me. The email address starts with "v", and is to a prodigy user. That's the only thing my email addy has in common with the intended recipient.

I've been on a campaign (read: tear) to forward each of these to abuse@yahoo.com (SBC bought prodigy, and couples with Yahoo somehow - sigh.)

I have a standard message I place at the beginning, basically chewing them a new one for clogging my inbox with junk that wasn't addressed to me. (Bad enough to deal with the junk mail that was sent to my actual addy.)

Can you tell this is a sore spot with me???? Is there anything else/better I can do to stop this onslaught?