Visit the individual topic and select: Options > Delete topic
You only see that option if logged in as an administrator or if you are author of the topic.
If you want to delete an individual post within a topic then visit the post and use: More > Delete post
NOTE: if that post is mid-way through a topic then the post remain within the structure but with a note "Contents deleted" in its body. It has to stay in the structure in order to have a hook on which to hang the subsequent posts and to assist with such things as "Threaded" view and "in reply to" notes at the top of posts in Classic view.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.