ACism - property rights enforcement

[BuddyQ]

Interesting this ACism, yet having read the sticky and some of the treads, I agree that property rights are paramount, but who enforces them? Say someone pulls a truck onto my crop I planted on 'my' land and starts harvesting? - do I just tell him to leave? And if he refuses and pulls out a knife? Do I use deadly force if it comes to that? Who is to say deadly force is justified or not, and by what authority?
If some private police force is engaged instead to apprehend the 'thief' what if the thief doesn't recognize, or has never acknowledged their authority and they start cuffing him and taking him away by force, why is the force they are employing justified, who gave them the authority to do that? Without some, minimal at least, state authority over the reasonable use of force to enforce property rights, doesn't this 'community' just become a jungle of 'might makes right'?