[John James Jacoby]

Won’t work. What would end up happening is your user tables would be out of alignment.

What do you do when that user WANTS to register at your WordPress site then? Just curious…

Best thing to do, is create a plugin on the bbPress side of things to add a usermeta value for that user that you can check on the WordPress side of things, and disallow access based on that usermeta value existing.