There really should be an option in the context menu when clicking on the Junk folder to empty the Junk folder, the same as there is for the Trash folder.  I don't like having to do a CTRL+DEL, sending the junk to the Trash and then going to empty the Trash folder.

In the case of IMAP folders e.g. Gmail, there should be an option to designate an IMAP folder as the Junk folder and for this folder to have the Empty Junk action and Icon.  This would then expunge the Junk mail on the server Junk mail folder as well as the local Junk folder.