Interactive

You could utilize a form of CAPTCHA ("Completely Automated Public Turing Test to tell Computers and Humans Apart"). These systems force a human-eye to read a randomly created selection of letters and characters and make the person re-type the characters they read. This system keeps users honest by preventing automated floods of entries via scripted or robotic code. In a well-designed CAPTCHA, characters are very difficult for automated systems to discern and read. In addition, tracking of IP address access speeds/frequency should be tracked in order to assist in throttling the rate at which any single user should be able to participate. Quality data centers will also provide a level of monitoring and defense against certain types of DDOS (distributed denial of service) attacks.