I was having a terrible time with comment flooders recently and even though the comments are put in the moderation queue, it is still quite a chore to delete or remove a hundred comments that are fifty lines long. Not fun by any means.
To create a simple solution to the problem, I have increased the timeout between the time a comment is posted and a comment can be posted again. By default, this time in WordPress is set to 10 seconds. I have increased that time considerably and added a really nice comment to the error code so people understand the reasons for not being able to post something. A commenter that is genuine is more likely to try to post somewhere in between the beginning of the timeout and the end. On the average she/he will only have to wait half the time set in the timeout.
I think comment flooders will not be willing to spend more than six hours on leaving comments on a single blog. It *should* be counter productive for them.
Though I am not sure this will work, I am going to give it a shot. I have another comment spam prevention script (or service) in the works but ETA on that is completely up in the air.
How do I do this?
If this is successful (looks like it stopped 2 sets of flooders already, I have a little script that tracks how many attempts were made) I might post this on the WordPress fora.