The quote should probably actually say “Private property is theft.” See, Marxists make a distinction between private property (which they consider to be unjust and illegitimate) and personal property (which they consider just). Personal property would include such things as one’s own body, clothes, home, small amounts of money --basically items intended for personal use. Private property, on the other hand, refers to ownership of the means of production, large amounts of capital, and large amounts of land.
Theoretically, in a real Marxist system the power to redistribute resources would not be given to a small group of people, but would be in the hands of the majority –the proletariat. It would be decided democratically. That’s the theory, anyway. Whether or not that can work in practice is debatable.
You are confusing private property with personal property. A person’s body is personal property not private property. Marxists believe that people have a right to have personal property.
There is force in Libertarianism. The force in Libertarianism is the force used to uphold and defend private property. To paraphrase James Madison, It’s the force used to protect the minority of the opulent from the majority who are less fortunate. That's to say, It’s to keep the poor from rising up and redistributing the wealth of the rich. Now you might claim that all interactions in a Libertarian system are “voluntary,” but it sure doesn’t look that way from a Marxist standpoint. The lower classes do not work for the rich voluntarily. No, they work for the rich because they have no other choice but to work for the rich or starve to death. People of the lower class do not have the resources to do anything other than be slave labor for the Capitalist class in a system that uses force defending the private property of the Capitalist class.
Edit: grammar.