Can’t keep new blogs from being created?

I had been using the signup question that I believed was created by Andrew, but I started getting more SPAM blogs so I decided to temporarily disabled registrations. They were still able to create new blogs so I deleted the wp-signup.php file and I’m still getting new blog registrations, particularly from a domain. I’ve now added this one domain name to the prohibited list to see if that works, but I don’t understand how new blogs are being created in my installation. I know this forum is just about supporting the premium plugins, but can anyone point me to what might be happening or the latest plugins/ways people are handling keeping blogs from being created.