Is it possible to ban a website during registration?

[sothis]

I'd like to ban a certain website from being inputted while a user is registering. Sort of like how you can ban subnets for email and then it won't be an option to use it while registering. Is there a way to do that with the website field? (or even, a signature? I could write something to parse the signature for the website text being involved, just not sure where I'd put it)

thanks!