This isn't a permissions problem. Excel is recording the paths to files that are being edited (and therefore "locked"). You have to find that file and delete it.
(It's possible that instead of storing a list of paths, all in one file, it's instead creating a "lock" file alongside the file it thinks is in use. Same concept: find the lock file and delete it.)
I don't have Excel (nor any other Microsoft product) installed, so I can't look to see which method it uses. I'd first look for invisible files in the same folder as the "locked" file, paying close attention to files with similar names. Failing to find any of those, I'd use defaults read to look in Excel's preference file to see if it's keeping a locked file list in there.
