dejudicibus on "User registration hook BEFORE inserting in database"

I am looking for a hook that I can override when a new user is signing up but BEFORE the user is saved on the database. That is, I wish to have the possibility to filter which user can ASK for registration depending on the data they provide.

I already have a function that allows me to authorize/deny new registrations. It is not that. What I need is something BEFORE that, automatically filtering request.