I would propose that Evolution included one of the following:

1. An option in Edit > Preferences > Mail Preferences > Junk that allows the user to toggle and option to "Delete Junk I mark as Junk immediately"

2. The right-click context menu include an option to "Mark as Junk and Delete"


Rationale:

While manually going through my junk to see if anything has erroneously ended up in the junk folder, I noticed a number of emails that I had marked as junk as they had ended up in my inbox and I had right clicked and 'Marked as Junk'.

It occurred to me that since I'm marking them as junk, I don't really want them in my junk folder, I just want my spam filter (in my case bogofilter) to learn that these emails are spam and then they could be deleted (rather than having to delete them later as a separate action).