We cannot allow registered users (that is, regular users) to have the option of deleting their own threads and posts. The issue here (and I am BY NO MEANS suggesting you're doing this - I'm only saying I've seen it done regularly by others) is that students who are trying to cheat can have a tendency to post something, get help on it, and then delete their thread so that their professor can't find it. Since we take academic integrity very seriously here at MHB, this is one way to make the job quite a bit easier on our moderators.
Now, we do allow trusted folks like staff and MHB Math Helpers and MHB Ambassadors to delete their own threads and posts.
The work-around is to use the report post tool and suggest to the staff via that mechanism that you would like a post or thread deleted.