There are a couple ways for a site administrator or moderator to delete a topic:
On The Topic ItselfThe first option is on the forum topic itself. If your account has permissions to moderate that forum, then at the bottom of each topic there will be four buttons: Delete, Move, Lock/Unlock and Split.
Just scroll down to the bottom and click the red ‘x’ icon, next to the rest of the thread functions, and you can delete the post.
From The Forum
The second option is on the Forum itself. At the bottom right of the forum (again, if you have Forum Moderation privileges for that forum) is an option to Moderate This Forum.