[1]You cannot have transferrable property rights if contracts aren't enforceable against third parties. Libertarians are split about more complex cases. Ask this hypothetical to a dozen Libertarians, you will get a wide spectrum of answers:
[2]"Fred and Jeff own competing sales businesses. Fred has a contract with all of his employees that if they leave him to work for a competitor, they may not contact existing customers for 180 days. This contract is valid and enforceable against those employees. Fully aware of this, Jeff hires away Fred's employees and directs them to contact their existing customers and try to get them to switch to his company. When the employees express fear that they might be sued or that they shouldn't break their word, he tells them not to worry about it. Fred hears about this from his customers. Does he have a suit against Jeff?"
[1]LOLWUT?
Which word didn't you understand?
[2]As I said before, being a dick isn't actionable. Breaking a contract is.
I understand that's your position, but Libertarians are in fact split on the issue of whether Jeff broke a contract or was merely being a dick. I just asked a bona fide Libertarian, and his position was that Fred has a suit against Jeff because his implied contract with his employees (that he would pay them money for breaking their contract with Fred) makes him an indirect party to the contract he asked them to violate. In a sense, they are acting as Fred's agent when they violate their contract with me. This makes Fred liable.
I don't know what my own position is. I find these kinds of situations to be very difficult.