Most of these machines have a manufacturer-universal key. These machines do not abide by automobile conventions for operation.

This is to say that the person who knows how to operate the machine also has access to the key regardless of whether a key is in the ignition. The standard for disabling machines like this is to remove the “night-key”, which disables the battery and therefore the ignition. The problem with that is the universal nature of night-keys.

Edit to add that a lot of forklifts operate pretty similarly to autos, besides the rear-wheel steering. The mini gradeall in the pic takes a little more knowledge to get moving but an average driver could figure it out in a couple minutes.