CloudFlare (https://www.cloudflare.com/ https://www.cloudflare.com/) does something similar to this. It will present a CAPTCHA challenge page to users who have

CloudFlare (https://www.cloudflare.com/ https://www.cloudflare.com/) does something similar to this. It will present a CAPTCHA challenge page to users who have had suspicious activity within a certain period of time, according to Project Honeypot. If you want to be really stringent (at the cost of some false positives), it will also challenge people based on HTTP header analysis, and with a paid account they will also protect against XSS and SQL injection in POST requests. They've done a fantastic job of cutting down the number of fake profiles on my client Set For Marriage (a dating site where it was a huge problem before), and the rest of their features are pretty good too (the asset caching has resulted in about 60% faster page load times).