Aaaaaah NOW I understand! 🙂 Thank you, that worked perfectly – this is exactly what I wanted.
Well, I changed the login and register URL and it works good with bbPress. Also, i use Antispam Bee (because akismet isn’t really according to the privacy laws in Germany) which helps, too. Third is WangGuard which stopps spam registrations and sploggers. Works. 🙂
Security (forum gets hacked, WP gets hacked) and traffic/performance, but also because I would like do keep my WP database as tidy as possible. But it would be nice not to discuss about that – I’m just interested if this is possible, and if yes, how. 😉