Nick I have sent you an email regarding this topic but I thought I'd post here for general feel of how other moderators react, when someone just sits on the forum and post on forum for hours spamming
I try to keep my cool and delete all posts but gosh if I had the means I would just block that IP.
The problem with blocking IPs these days is that it doesn't work too well. There are so many ways of getting around IP blockage that it has become almost useless unless you block everything - then you'll be blocking legitimate posters too! Arrrghh! That's making an even worse problem.
I agree with the other poster that once you have a spammer it's hard to get rid of them. The most important thing is not to make the situation worse. I've managed and looked into many forums over the years and have come to the conclusion that most (not all) spamming is a reaction to poor moderation. By that I mean that the moderator somehow incites one or more of its visitors in some way. Good moderators learn how to quickly diffuse such a situation while bad ones tend to create even more incitement/excitement which results in even more spamming.
There is a delicate balance a moderator has to attain in communicating with forum visitors. On one hand you need to remain aloof, on the other hand you need to be open to dialogue when a situation threatens to get out of control.
Most spamming, if dealt with early, can be diffused so that the spammer never feels compelled to resort to unusual means (IP changing, etc) to circumvent the moderator's blocks and continuing spamming the board.
Moderators that don't want to become proficient and skilled in dealing with their little monsters will resort to making the forums user only. While that is good, it tends to greatly restrict traffic and activity on a forum, making it seem dead.
Hope some of these experiences and tips might be helpful. I hope you can solve your problem and have a productive forum.