Deleting is very often abused as a option for solved topics. I wouldn't grant normal users the ability to delete their own topics or posts. I would also implement a version-based timeline for topics to guarantee the integrity of a discussion stream and to prevent the nulling of posts.

Greetings. If you wish to delete your own post, just edit your original post and state that you wish it deleted. A member of the staff will do it for you. Thank you.