Random Poll Question

I am at a total loss. Within my webpage, I am using a call function for a random poll as below .

It works fine and the script is great. Is there a way to ensure the called "random" poll is not one that has already been voted on (using the cookie)?. I am sure the answer is easy, but I cannot figure it out.

I should add, basically, I want to use the cookies even if temporary. I need it just to work during the actually session the visitor spends on the site. You see I have a series of random polls, but I don't want to have the visitor (on the same session) to see polls they have already completed, I want it to skip only to uncompleted "random" polls. Does that make sense?

Any takers on this?