if you are using a database, then use the voter's I address as the key, and declare (in the database description) that the key be unique.

this isn't really the best idea, though--you'll shut out (potentially) tons of visitors that come from the same IP, such as a proxy server (ahem, *AOL*, ahem) and allow the same user to vote if he is coming from an ISP which uses dhcp to assign addresses.