Are you talking about a discussion here on this community or your community you manage?
If someone has replied then I would think that would be the reason you couldn't delete it. Did the person answer your question? Is the question potentially valuable to someone else? I'd leave it if you can say yes to either of those questions.
You could always mark the item as abuse and reason why you don't want it there. I'm of the belief though that if someone responded it should be deleted because that takes away from the person who took the time to respond to you.
Hi Kristen Gastaldo,
Reviewing through the case submitted in your customer group I see the specific discussion was one created here in the Jive Works community. Since there were replies, an admin needed to delete to discussion, which has now been actioned on.