How to exclude some mail servers ?

[Jawelin]

I would avoid new users become registered with mails from certain servers.
How could I apply some simple hack (and where) to create not allowed substrings to search the mail for, when a new user subscibes ?

Thanks

[Reeve of shinra]

I think you can 'ban' certain email addresses in the admin panel. That what your looking for ... something so that anyone with an aol email cant use that to register?