There, I'm having a mystery behavior on my site. There

Hi There,

I'm having a mystery behavior on my site. There is a site on the network, dcrwheels.co.uk. In network admin, when I look at Sites and check out it's user's, it is signing up over 100 new users an hour. When I go to the actual site (dcrwheels.co.uk/wp-admin) and look at users, I only see the admin user - no other users.

Two questions - one, why the discrepecy, and two, how do I stop all these spam user signups?