Delete email folders

Using macOS Mail, I noticed that there are two folders for junk email, namely "Junk" and "spam". I could not delete either of them from within the app. Furthermore, I could not delete any folder from within any of the webmail apps (Horde, Roundcube, SquirrelMail) either. So my questions are: 1) Why are there two folders for junk mail? 2) How can I permanently delete unwanted folders (I accidentally created a third unwanted folder)? Thanks!