New user registration disabled but users have registered

We've decided to go in a different direction from our original plan and have decided to turn off new user registration. But somehow, a few people have managed to register as new users anyway!

I have clicked the "Registration is disabled" button. I have also listed several email domains in the banned list but this seems to do nothing as these users are also registering with one of the banned email domains.

I have clicked on all the links on the site that have to do with registering and I always get the "User registration is currently not allowed." message. if you want to try.
Any ideas on how this could be happening?