There are two ways to delete posts. At this time, only instructors (that is, professors and TAs) are able to delete posts.
The first way is to first locate the post in the Q&A feed on the left side of the page. Hover your mouse over it so that the downward arrow appears. Then, select "Delete" from the dropdown menu:
The second way is to open the post and hover your mouse cursor over the body of the post. This will cause the "Actions" menu to appear:
Hover your mouse cursor over the menu so that the dropdown menu appears, and then click "Delete question":