Setting auto-close to 1 minute would be quite an effective solution except I’ve found and installed the voting plugin and closing a topic also prevents any votes being cast for it.
Auto-deleting replies after a minute would be another way of cleaning up the sub-category but as far as I can tell that’s only possible on a topic-by-topic basis, not at category level.
It looks like sending all replies for moderation is the only viable option at the moment. Unfortunately, they can’t be redirected to a different category at the same time, which means either deleting them (and irritating users who, quite reasonably, would say they shouldn’t be able to hit the reply button in the first place) or accepting, then splitting/merging the reply in to another sub-category - more work for me but a slightly better user experience.
Webhooks and CSS are the undiscovered country at the moment and I want to launch the competition over the weekend. I can look at it retrospectively though. This would be a good excuse to broaden my knowledge.