[undyingtrillion]

Ban IPs. If they are largely coming from a country you are not marketing in, consider blocking off that country as a "stop the bleeding" measure.

See what other patterns you find and ban IPs based on those.

Edit: I also remember people putting in fake fields that are not visible to the user. If they have any value in the form submission they are scripts and can be rejected.

[anon8588]

Thanks. I am looking in how I can throttle sign ups by IP address without causing problems for legitimate users.

I've yet to try the "hidden fields" honeypot.