I have a contest site, I want it to prevent people going on it when they have submitted the form on the page. Is this possibe?
Thanks
You could build system where users are required to enter their email address, and enter a verification code which is sent to them. But they would only be able to enter this verification code once. Therefore, the only way of cheating would be to use two separate email addresses (and they would have to have access to both).
Cookies would not be the best option for this as anyone could clear their cookies.
It's not necessarily possible to stop users requesting a page, however you can either redirect them to another page or show some kind of error message to them, to tell them that they can't resubmit.
Presuming you have some kind of user registration in place already (and are tying competition entries to those users with an ID) then all you would have to do is check for the existence of a competition entry in the database and then, if one is found, either force a redirect (likely in <script> tags) or replace the rendering of the page with the notice that you can't resubmit.