In case you have the time for it:
I tried the "prevent user from registering"-option just now and it seems as long as the user is a member of any allowed user groups he/she is able to register no matter what option I choose in the profile field.
I'm pretty sure I have the correct field name in the configuration so any chance this is generated in the back end? Yes, I am probably the one doing soemthing wrong ^^ Any way, if you got the time I would appreciate if you checked this out.