Deletion of the posts / topics for admin is built-in of course. I think it's obvious when you are logged in as admin: click "Delete" link near each message to delete the message; click "Delete topic" link, located at the bottom of each topic page.
Small note: you will need to delete the whole topic if only one message posted in it.
Admin is allowed to delete anything anytime; moderators are allowed to delete the content of the related forums only. Users are allowed to delete their own messages if it is allowed in options (RTFM
for more - $userDeleteMsgs