Also, on the list of topics, and I mean on the "New Posts" page, you can see the thread titles and the first five tags. Now remember, a tag is a forum. So click on a tag that strikes you ad graphitti-like. Top left of the page are two choices. "Add forum" and "Delete Forum". Click delete.
I found a "Thread" forum. Sure enough, it had some threads in it. And sure enough, I deleted that forum. You might be a little careful about what you delete, though. You might miss out on some good stuff if you get carried away. By the way, when you delete the forum, you're only deleting it for yourself. For everyone else, it's still there.