I was unable to find anything but outdated information on this issue. I'm using vs 1.3.8a - php4 server

I had a person subscribe on my site yesterday - on further investigation I discovered that the e-mail address used appears to be a virus serving hacker from what I'm seeing in my search for the url. How can I ban that e-mail url from registering on my site?

gift-mailboxes.com is the url they are using - the searches I found appear to be different variations of Constantine@ .