A lot of good comments, but they all boil down to the same thing: "You can't get there from here." Re-examine the need for "one time only". These people here can write perl to stuff the PM ballot box. They don't in general because: 1) It would prove nothing. 2) They are interested in the untarnished results. Myself I vote for cookies. If I really go to the trouble to delete the cookie (after finding it) and go back to your site and vote again... how much damage have I done? How much have I skewed the results? Suppose I take 15 minutes and vote three times. Big woo? The only sure proof way (and there is no sure proof way) is to require registration, which you opted out when you said "anonymous".