Thank you Oyabun1
Have seen the KB on Custom Profile Fields.
The goal is to just prevent the known spammers access to make a post in forum, and make less hassle for the normal signups.
This is a way i only use what i have to day regarding custom fields, not a house full of pad locks.
CAPTCHA is normally good enough, but it it will be broken as soon "they" also upgrade or update their software.
A small patch in user_registration_code to look for the "bad words" in the "custom profile fields", and "pretend" that all is OK and no feedbacks to the bot, -is the way to go. -Let it wait forever to get the activation-mail.