Fake Registration Exploit WPMU

Hi,

I just recently changed my wordpress setup to WPMU.

Since then (2 days) I've seen a few fake registrations popping up (getting the email notification).

But the supposed registered user is not showing in within the admin on either of the sites.

How do I go about preventing these fake user sign ups. It seems specific to WPMU (I'm assuming).

I'm currently using: Wordpress 3.6