I know one ISP in Britain (AOL) that a couple of years ago had a block on ALL mail from Belize. After I complained it took maybe a week before it was resolved satisfactorily - but it was.

You have to understand that spam accounts for an enormous proportion of the load on major servers worldwide. Until they find some way of either preventing it being sent or intercepting it en route I can understand why they want to block IPs that they know produce a lot of it. I read somewhere that most spam (>85%) is produced by about 6 people, mostly in or associated with Russia.