Spammers will do as they do here, so long as they are able. They are not going to stop. Banning IPs is only effective if they use a static IP.
There are ways to limit how many posts people can make in a certain amount of time. Depending on what back server Forum you are using, there should be other ways to mitigate this behavior.
Prevention is best, deleting multiple posts after it has happened is required but stopping it before it happens is best.