Lockout

You could use a "lockout". A lockout is a device used by Electricians, that allows them to lock breakers in the off position. When an Electrician is working on a circuit, they will turn the breaker off, place the lockout on the breaker, place a padlock and a name tag through the lockout lock hole. This prevents others from accidentally energizing the circuit, while the Electrician is working. In most places, the Electrician is the only person with a key to the lock, and the key is kept on them at all times.

Disconnect

An "electrical disconnect" or "safety switch" is a more permanent solution. They are available in fusible and non-fusible varieties, with a wide range of current ratings. They usually have the ability to be locked in the off position, using a common padlock.

You'll wire it up to interrupt the ungrounded conductors of the feeder to the panel.

• Turn off the breaker in the service equipment panel that feeds the panel in the garage.
• Disconnect the feeder wires from the panel in the garage.
• Mount the disconnect near the panel in the garage.
• Connect the ungrounded conductors from the feeder to the line side terminals in the disconnect.
• Using a length of the same gauge cable as the feeder, connect the ungrounded conductors to the load side terminals of the disconnect.
• Using appropriately sized twist-on wire connectors, other approved means, or terminals in the disconnect if they're available. Splice together the grounded and grounding conductors of the feeder and the short length of cable.
• Terminate the other end of the short length of cable in panel.

Now when you move the disconnect into the off position, the panel in the garage should be dead. You can then lock the disconnect door closed, and the handle in the off position.