I'm myself an Anarcho-capitalist, but I have one doubt:

What if at least one of private defence agencies (for example - agency A) - which are an essential element of the Anarcho-capitalist concept - after aquiring right amount customers - by strictly voluntary/free market actions (by serving better services for lower cost) - would:

• order its customers to pay more - under serious phisical penalty;
• forbid them to use the help of other agencies - under serious phisical penalty;

Those actions will probably result in:

• the deterrence of other customers
• resistance from (enslaved) already "owned" customers
• hostility from other security agencies - which will not normally be able to compete for (enslaved) clients of the "A" agency.

But what if ... the "A" agency will be able to overcome all of the above obstacles?