[Smush Pro] Remove original images after smushing

How do I remove the original images that Smush has compressed?
I have a certain amount of space on one of the websites I manage and after Smushing it is nearly at the limit. It was close to the limit (75%) before Smushing and now it’s at 97%. I would like to remove the original images to free up space. I was under the mistaken impression that this would happen automatically for resized images.
Unfortunately, I cannot simply ask for more space on this site. Is it a case of deleting all image files with .bak in the filename?
I know this will remove the ability to restore the originals.