Post Voting Plugin - In case of refresh page, you can vote again

Hello sir,
i'm working on an old website. The last dev used your plugin last year, so i've just paid to renew the licence. I'm retrieving the same issue of last year.
1) We changed the thumbsup icon (up.png) with a classic button "vote" design
2) We added the [wdpv_vote_up] tag into the product page.
3) when user click on the vote button, correctly is shown the button "you voted"
4) if we refresh the page with F5, the button "vote" is shown again.. so user can press it again (but no more votes are added into statistics).

We would like that, after one vote, there is no possibility for that user to vote, because he can leave one only vote.

Can you help us?
regard
antonio