If you're allowing everyone to vote, then only one vote per IP can be cast. It doesn't track usernumbers. If you're allowing only registered users, then it'll track by usernumber.
Polls haven't been done with UBBCode since version 6.4 (or maybe 6.3) they are created using separate scripts. They cannot be edited.
The user could set a time period though when they created the poll, if the time period is past, then no more votes can be cast, no matter who you are.
