Osu on "Disabling wp-login.php register form"

Hi,

I've been getting a few spam sign ups lately and trying to think about the best way to disable them while still allowing sign ups via a plugin I'm using called 'Profile Builder Pro'.

Is there a way to use functions.php to literally not return the register form here? http://www.domain.com/wp-login.php?action=register

That would mean people could only sign up via the front end with the plugin.

Anyway tips really apprecaited.

Thanks,

Osu