Kill Comment Spam One Bot at a Time
IO ERROR explains how you can punish a particular form of bad behaviour.
“The sophisticated link spammer technique in common use now is to use some sort of script to harvest comment forms from a group of sites, then to fill in the fields appropriately, and a few hours or days later, to use a network of open proxy servers to relay the spam comments to thousands — or hundreds of thousands — of sites which use the same type of software. Repeatedly.”
His Bad Behavior plugin is a great first line of defence against automated comment spam. I recommend it.