Hi, need advice
Call a locksmith.
Explain to locksmith.
Pay locksmith to do locksmith things.
It’s just a regular door, not a sliding door
This locksmith recommends just a regular lock not a sliding door lock.
Any foolproof way to prevent this from happening?
Remove the door.
Keep a hungry bear on the balcony. I bet the tenants won’t ever use it.
Evict your tenants.
Burn the building down.
Create an elaborate plot in which you gradually feed high quality intelligence to the security services about a foreign government using a nearby building to train terrorists. Ensure that it’s a building that can only be seen from your balcony. The security services will commandere use of your apartment for surveillance and your tenants will no longer get locked on the balcony. The feds might though, so best go with the bear.