You really should not have multiple people opening the same shared front-end file. Each user should have their own local copy of the ACCDE/ACCDB front end, with the tables linked to the shared SQL Server back end. A shared front end causes corruption and locking headaches eventually.

For tracking who is in the application, I would use a LoginT or UserSessionT table on the server. When the front end opens, add a record containing the UserID, computer name, login time, and a LastHeartbeat time. Have a form Timer event update LastHeartbeat every minute or two.

When another user starts the database, check that table for active sessions. If someone is active, display a message such as:

"David is currently using the database. Do you want to continue anyway?"

If you truly need to force someone out, add a ForceLogout field or LogoutRequested timestamp to the session table. The current user's Timer event checks that field periodically. If it is set, display a message, close the forms, clean up the session record, and quit Access.

You cannot reliably "kick out" another user's Access session directly from a different copy of the front end. The existing front end has to cooperate by checking the shared session table and shutting itself down.

Also, do not rely solely on a login record, because Access can crash or a PC can lose power. That is why the heartbeat is important. If LastHeartbeat is more than, say, 5 minutes old, treat that session as inactive and ignore or delete it.