Can visitors vote more than once?

By default, yes. Each time someone submits the poll, their vote is counted.

To limit each visitor to one vote, select the Poll block, open Answer settings and turn on Allow only one vote per device. OpinionCamp will then refuse a second vote from the same browser.

This is a per browser limit, so it is not a perfect lock. Someone who clears their cookies or switches devices could vote again. See Allow only one vote per device for more.