I forgot to mention what the current workflow is to deal with spammers.
- Delete the post
- Delete the post permanently
- Ban user
It is normally 1 spam post per user. So the higher the number of posts are the workflow becomes tougher.
IIRC there was filter plugin which would not allow to post at all if there is a banned word. I am happy that there is a ban IP plugin. But in this case, something like ban IP range is more useful.