Nonsense.
Saying you have the right to do something and actually having the right are two different things.
Just because a person steps on a piece of property does not mean they surrender all rights to their body. A conflict of property rights has been created, but that does not mean that one right negates the other.
So your argument that a person only has as many rights as the owner of the property allows is false. A person has full ownership over their body, irrespective of location, that is proportionally reduced by committing aggression.
Rothbard’s whole point, that you clearly missed, is that you can’t do anything you want to someone just because they are on your land. Your so called contract is a merely a sign disagreeing with that simple fact.
I have to wonder, would you defend the enforcibility of a sign that read: “Anyone who walks on my lawn becomes my slave for life”? After all, it fits the template.