Lack of interest users must set username different from password


Active member
Hi, I have an annoying user on my board that has nothing to do, and in his spare time tries to login to other users accounts.
He understood that some users just set the username as their password.

To prevent this to happen in the future, is there a way (during the registration or after) to force the users to choose a password that is different from the username?
And maybe force them to use letters and numbers?