restraining users from taking poll more than once

I would like to know how I could restrain users from taking poll more than once. I did some search and found that it might be possible though locking ip and cookies..and I dont want to go by either route. The next thing that I think is of recording the poll number and user name in to database once they take the poll and control it this way. but this way of doing might become a problem as there might be many user and many poll down the line and this database would become very vast... Is there any other way to do it.... ...