Moderate comments on specific posts/categories

Hi there,

I have been searching high and low for an answer to my question. Maybe you can help?

When I run competitions at my blog, I generally want the participants to give me the correct answer (or whatever) to the question in the comments. Now, I would love to be able to moderate comments on just my competion posts (so that readers can’t check the other answers before posting their own). Any way to go about this? A plug-in? Something? I don’t want to moderate all comments on all posts (akismet does that very well) – just the comments on specific posts.