Well there never has been a stateless (post-state) society so there are only theoretical answers.  If most of your customers are law abiding they may choose to not support your unlawful (in natural law sense) actions against another.   The question also becomes IS killing a competitor lower cost and lower risk then arbitration, licensing, marketing, or other non-violent means.  I mean trying to have your competitor killed could get you (or someone you care about) killed.  Once again all theory but if private security forces replaced police states then you could find your security contract revoked per a condition in the contract.

Still I wasn't really talking about the lack of a state. We have never had a post state society so it is all theoretical and any theory is just that.  I was just pointing out the power of the cartels is a direct result of regulation, namely the prohibition on certain substances, not a lack of regulation as you seem to claim. 

The US government through ineffective regulation created the cartels and the criminal violence problem in Mexico.  The cartels and the power they have simply couldn't exist without the massive profit margins and massive barriers to entry created by the actions of governments.  If governments wanted to they could end the cartels tomorrow.  As an unnatural market they simply can't exist without the continued support of governments.