When it comes to Q&A in general, I am very much against:
Disallow downvotes, upvotes are enough to determine winners, and downvotes can hurt people's feelings.
as that turns the whole world into touchy-feely-everyone-is-wonderful, which, unfortunately, is not the case, and basically to me seems very fake and, in the end, counter-productive.
That being said, contests are not about "is this a good vs. great vs. possible-even-bad" answer, but rather ranking of preference. Much like voting in an election. And just as Robert's Rules of Order normally allows For/Against motions in general but for elections only allows positive votes (so if there is only one candidate, one vote is all you need to win - nobody can vote "against", whereas if there are multiple candidates, a majority (or other specified amount) is needed to win), doing the same here actually does make sense.
As far as:
Some way or marking winners, used to be done by accepting the top answer after the contest was over.
that is an interesting idea. Specifically for contests, more than just "one" winner makes sense. The simplest solution is just to rank by score and freeze voting at the end of the contest. Then the natural score-based ranking takes care of everything.
For the implementation, I think this could be simplified to Category-specific settings to:
- Allow Upvotes only (vs. default Upvote/Downvote), though should allow someone to un-Upvote.
- Freeze all voting, either at a preset time or an action by OP or by a moderator.