Problem with "Junk Users" registration

hello_world

Hi there,

I have a problem that there are "junk new user" registrating a lot of time everyday.
They now account for 25% of the total user.

These users never post anything. They registered using "meaningless numbers" as "users name"
and they all registered using email address from a particular provider (eg. XXXXX@abc.com), some of them are from similar IP address. (I have already set to forbid some range of IP address to double registrate within a certain period of time.)

I have already set the validation question and validation code to prevent spam.

How could I prevent these user from registration? eg. forbiding the use of registrating using email address from a particular provider?

Any other thoughts?

Many many thanks.