This is the way I did it but does require the use of a USB stick formatted to FAT32.
Plug in the USB stick and go to Settings, System Settings, Storage. Find the drive your import saves are on and highlight it, now press the Y button. Pick the USB drive to transfer to.
Now the transfer saves will be under other or system items, you'll recognise them as they will have Shepard's first name on. Make sure to deselect everything else you don't want to get rid of.
Once that is done, move the transfer saves to the USB stick.
Take that USB stick to a PC and you can delete the Xbox folder that was created (you'll need to turn on view hidden items) or simply format it. Those saves are now gone for good.
