As long as you are sure there is no one currently using the database, you can delete the Access lock file.
I haven't used Access as a Backend for quite sometime, so I'm relying on my fuzzy memory. There are times when a system/windows filelock gets orphaned and maybe this is what you are running into. When this happens, Windows thinks the file is in use, but it isn't. Whoever was using the file somehow didn't let Windows know it was done or Windows ignored them. This can happen to pretty much any file that is opened for edit; a datafile. So in my fuzzy experience it happened with Access from time to time, both the lock file and the database. This wasn't a surprise since it would coincide with the Access lock file getting out of sync which will happen pretty much regardless of the precautions short of moving to SQL Sever.
So when a file gets an orphaned file lock on it, the easiest and surest way to unlock it is to reboot. Isn't that the answer for everything? If it's on a FileServer/FileShare you may not want to reboot as that can be disruptive, so something you can try is to rename the Access Lock file. Every once in a while this will work. The Windows file lock might be dropped or it may stay with the renamed file which you can delete on the next reboot. Either way you can get back to work on the Backend Database. If the Rename trick doesn't work, you might try the techniques here:
http://www.howtogeek.com/128680/how-...es-in-windows/ I've never tried it, so if you do, let us know how it goes.