Using the MONITOR utility at the server console (LOAD MONITOR.NLM), find "file open/lock activity", then drill down to the file in question. This will tell you the connection number of the user that has the file open. Press ESC back to the main MONITOR menu, then choose "connections", and find that connection number (you can press F3 to sort the display).
Once the offending connection has been found, you can contact the user to close the program, or press the DEL key to disconnect the user.