Just forget to put the key in the lockbox. Or put a different key in. If they only do use it for emergencies they will have to decide while at the site to allow a fire to burn/crime to occur/injured person bleed out because the key isn't working or just break down a door. If they try the key in a non-emergency situation they will have a very hard time explaining it.
I would also consider placing a tamper evident seal on the key box to see if anyone had accessed it.
Just forget to put the key in the lockbox. Or put a different key in. If they only do use it for emergencies they will have to decide while at the site to allow a fire to burn/crime to occur/injured person bleed out because the key isn't working or just break down a door. If they try the key in a non-emergency situation they will have a very hard time explaining it.Good thinking, except it doesn't factor in incrementalism.
Once the custom gets established, the fire chief will be doing inspections and checking to make sure your key is the correct one. If not, you will be notified, threatened with forfeiture of service, threatened with a fine of some dollars PER DAY, and subject to reinspection. This will all be upheld by courts because after all, if your house burns, your neighbor's house is imperiled.
Much better to nip it in the bud.