the lock file may contain enough information to enable you to figure out who has it open, but even if it does, that requires a phone call with no guarantee that they will answer. You could know who that is if you logged every user in, password or not. That only requires that you grab their Windows login info and match their Login ID against your table of users. You could also code to terminate their connection by setting a table flag that a hidden form's timer event looks for and start a log off routine. That might cause data integrity issues if you terminate a connection while someone is in the middle of a record edit.
Regardless of what you do, there is the widely held view that multiple user of 1 front end is a recipe for corruption and isn't a matter of if, but when. That might be a problem in waiting that can be more damaging than what you're faced with now.
EDIT
I should mention that if every user has their own fe, it doesn't prevent you from making development changes. When you release a new version and have each fe verify it is up to date, users are typically forced to update rather than be allowed to use an outdated version. The effort it takes to force a user to download is typically worth the problems it prevents - updating fe's being one of them.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.