如何阻止人们故意重新提交表单？

I'm building a website using Ubuntu, Apache, and Django. I'd like to block people from filling out and submitting a particular form on my site more than once. I know it's pretty much impossible to block a determined user from changing his IP address, deleting his cookies, and so on; all I'm looking for is something that will deter the casual user from re-submitting. I'm building a website using Ubuntu, Apache, an