How to permanently delete back up files?

I’m trying to delete specific files from one of the back up folders because I don’t want to delete the entire back up. However, when I delete a file, it just reappears again. I shouldn’t have any viruses (just did a norton security check) and I have all the permission needed to delete stuff (I’m the only user for this laptop). It’s a Lenovo with windows 7. Anyone have any ideas?