Can a thief disable an apple air tag?

Bottom line, AirTags were never designed to be used as a theft deterrent. They are for finding lost items like keychains, luggage and the like. Just like any other tracking device if a thief finds them they can disable them, period. And thieves know all about AirTags and GPS tracking devices. The first thing they will do after stealing your UTV is search for, find, and destroy nay tracking device they can.

Yes, when they hear it beeping because you left it behind or when they receive the anti-stalking warning on their phone they will be able to disable it.

sure, if he/she opens the air tag and take out the battery it will be as disabled as can be

No.