it's fairly simple, actually. i worked for a company where i was the manager of the web support team. you would be surprised how much information is provided about your computer/os/browser/location every time you visit a website.
we used Akamai to track user information.
all of the RSVP's would come from the same IP address only with different names in the body. you scrub out the cheaters using an online bot, and let the real people come through.
a home grown app would be a little more difficult to identify.