You can't.. Only the moderators can delete threads. And it has to be for a good reason. By the way.. What space are you talking about? You only have less than 50 posts.
You really think you have too much? Seriously? Imagine me! Over 24,000 posts and several hundred threads I have created over the years..
I have over 20,000 posts. That's a lot of pages. You can unwatch threads but you can't delete them. I can't believe you're serious about all that space. Oh well, here's one more thread to add to my list.