A better solution is to let them register, but check all registration/post IP addresses against the blacklist. If they match, unobtrusively move them into a usergroup that just seems like your site isn't working that well.
Reddit uses a similar system called shadow banning. When a user is shadow banned, they think their content and submissions make it through, but they actually are hidden to other users. On rare occasions, real users can accidentally get shadow banned; this is not ideal, but shadow banning is effective enough that the sacrifice is made.
Reddit uses a similar system called shadow banning. When a user is shadow banned, they think their content and submissions make it through, but they actually are hidden to other users. On rare occasions, real users can accidentally get shadow banned; this is not ideal, but shadow banning is effective enough that the sacrifice is made.