An idea, not sure how easy it would be to implement.
Add a "button" on the first post of a thread named "SPAM".
Anyone seeing a spam post just clicks on the spam button, and the post is automatically moved to the spam thread to be reviewed by a moderator.
If it is spam, no one else will see it and it get deleted and the poster gets their account deleted.
Sadly, this won't prevent them from coming back, but this "could" keep them from doing anything else.
If it is not spam, then it can be restored to it's original location.
It's seems the recaptcha isn't stopping them.