I don't think that's possible.

However, when you're doing this so often that it bothers you to confirm it, you could do it automatically on exit.
You can empty the trash automatically on exit via:
Account -> Properties -> Options -> Empty trash folder on exit
Or you can do it by setting the folder properties, this works for all folders and it's the only way to do it for the junk folder:
Select your folder and then via the menu:
Folder -> Properties -> Max number of stored messages
and select both On exit: 'Remove old messages' and 'Compress the folder'