If nothing else you just don't include the tag or title they are using for the challenge/contest and just make it its own thing. Which can be fun as well. Quite a few times those challenges the person who started it might not even be around or even be checking out most of them.
When I was doing some of the the color challenges for a while (I really should do one for a game) I was just creating very strange/weird short stories that included objects that where that days color. Everyone else was taking photos of flowers and most time the same kind.