Edit you poll and in "View Results Options" set "Views Results" to "After Vote" and save.
Then go to "Templates" and edit the template you used for your poll.
In "Template After Vote" at the top add
<div id="yop-poll-container-error-%POLL-ID%" class="yop-poll-container-error">Sorry, you've already voted!</div>
then delete these lines
<span class="pds-feedback-per-%POLL-ID%"> %POLL-ANSWER-RESULT-LABEL%</span>