Consider you live in a private community area where it's decided to hire a private policing service, with its SOLE purpose of making sure that the Non Aggression Principle is upheld. There will be a large demand and also a wide range of such services, which means great competition which in its turn will drive up the quality. If the service they provide is substandard the community will simply change service provider. Furthermore, security officers are private police. They monitor and protect buildings, shopping malls, and residential areas. Already today there are more security guards in the US than there are state police. In other words, more than half of policing is done privately today. Why couldn't it be 70%. Or 80%. Or 100%?
The starting point of my argument is however primarily a moral one. Being forced to pay for services you haven't ordered is coercion. Taxation is theft.
Which would be fine in principle, but throw in the local gangs forcing out legit competition for said service who's going to stop them. Or only government approved outfits again back to g4s for police, ouch.
Who's going to stop them? A heavily armed and pissed off public who are acting within their moral and legal right to defend themselves.