The only practical way I've found to restrict the reuse of web-based forms without requiring user registration is to log the apparent IP address. If your site is not very popular, this works perfectly well. On low-volume sites, difficulties generally only occur if everyone who visits your web site does so from home and comes to you via the same large, consumer-oriented ISP.

The problem becomes more complicated when your site actually becomes popular. I recommend reading the comment on a typical Slashdot poll results page: "This whole thing is wildly inaccurate. Rounding errors, ballot stuffers, dynamic IPs, firewalls. If you're using these numbers to do anything important, you're insane." This is the most practical advice I've ever seen on Slashdot, and instructive to all of us who aspire to create a popular web site.

Dave Aiello
Chatham Township Data Corporation