I spent a few hours trying to unlock an OpenOffice file that was "locked for editing by an unknown user" using a number of strategies including Terminal and my Administrative account. There are a number of obstacles to each of these strategies. I was able to identify the hidden files but unless you are very good at this sort of thing, gaining access to the needed disk images and deleting only the desired files will be difficult.
I would suggest the simple strategy I finally used which is a lot easier. Open the locked file in 'read only' mode and either rename it or save it in a new or different directory. Since I needed to keep the same names for the filename and folder, after saving the files into a new folder, I deleted <command + delete> the original folder and that also deletes the offending hidden file.
Then I simply renamed the new folder and file with the old names and everything works fine. OSX wisely, I think in most instances, makes it difficult to delete hidden files. In this instance, I believe my problem was brought on by unmounting a disk image containing a file that was being edited, so the "unknown user" was actually me. I will henceforth be a little more careful about closing files and mounted disk images.