Require registration. Block VPNs (as the people doing it are more likely trying to hide their true IP). If someone wants to use a VPN, require documenation. Be open about your limits.
But to be honest, the people that are abusing your system, more then likely they are doing it for financial gain, so it's likely their full time job, and they won't stop.
But to be honest, the people that are abusing your system, more then likely they are doing it for financial gain, so it's likely their full time job, and they won't stop.