A list is fairly worthless. It may be today, but when those providers clean up the server, it's now outdated.
I used to ban IP's. It turns into a high maintenance task, requiring constant attention (and subsequent frustration.) Now I only do it if the requesting IP's are not relevant. E.g., if a company only sells to North America, then they don't need the grief of some of the trouble spots in other parts of the world (see how well I avoided naming names? Hey, it's Monday, I'll take that self-gratification. :-) ) In this case, I ban a whole class of IP's in a one liner nuke.
What I found works far better is the logic behind spamming.
1. Spammers are paid on total delivery. It doesn't matter to who. If they can send out 1 million emails that don't bounce, or post to one million vulnerable blogs without getting the links removed by the time the check arrives, they get paid.
2. If a form can be abused, it will be.
3. If it's too difficult to abuse a form, they **WILL** move on to greener pastures.
Summary: make your forms too difficult to abuse, without making it more difficult for your users (e.g., captcha and other challenge/response fields are an absolute last resort.) It doesn't matter how else you do it, but this works, everyone is happy: your users, your clients, you.
there are many ways for visitors to add content, without login.
This may have been an attractive way to get initial content, but I suggest you now change this method. The ability to prevent anonymous posting is your front line of defense.