Find the post you want to delete, click the 3 dot menu below the post, select "Edit or close topic," scroll to the bottom of the the edit form, and click Delete; then confirm the delete. The post and comments on that post will be immediately deleted.
If you decide you don't want to delete the post but don't want to receive any more comments, you can just close the topic. Once you've selected "Edit or close topic," scroll down the page and uncheck the "Uncheck to close topic discussion" box. You can re-open the discussion at any time by checking the box.
Slipstream
Find the post you want to delete, click the 3 dot menu below the post, select "Edit or close topic," scroll to the bottom of the the edit form, and click Delete; then confirm the delete. The post and comments on that post will be immediately deleted.
If you decide you don't want to delete the post but don't want to receive any more comments, you can just close the topic. Once you've selected "Edit or close topic," scroll down the page and uncheck the "Uncheck to close topic discussion" box. You can re-open the discussion at any time by checking the box.