Who voted in a poll - disable
 

...

You can edit template newpoll, search for make_votes_public, and remove the code around it (everything from <li> to </li>).

A user could still create a private poll by messing with the form information that's submitted, by they'd have to really want to do it (and you'd need a plugin to guard against it).

Go to the admincp, on the left under "Styles & Templates" click on Style Manager. Then on the right, next to the style you want to edit (or Default if you have no other styles), select "Edit Templates" form the menu. Then scroll down in the lest of templates until you find "New Posting Templates" and double click to expand it, then double click newpoll. You can enter make_votes_public in the "Search in Template" field to find it.