Simple cookie question

This may be a stupid question, but I'm trying to set up a system where I can take a poll from visitors to my website and then set a cookie so that they can't vote more than once (until they clear their cookies at least). Problem is, I don't want to put it at the top of my page, because what if somebody opens up a poll but decides not to vote, then later changes his mind. The cookie will prevent him from voting. I want the cookie to be set right after the user clicks the vote button, but every time I do that, I get an error about headers already being sent. Is there any way around this? Thanks