Let us say I have a mansion in the Hamptons. I'm feeling generous and decide to give it to you, but on a condition: I may have the use of it 10 weekends a year.
Certainly there is nothing unjust about me coming to use the house on those ten weekends, correct? In fact, if you try to deny me the use of the house, it would be you who is in the wrong, and I would be justified in using the law to make you to meet your obligation to me. And it would be utter nonsense if you claimed that in doing so, I was "stealing" your property "at the point of a gun."
Well, property rights are a gift to all of us from the civil order that we found pre-existing us upon our birth. As Rousseau notes, outside of civil society, property rights do not exist, and all we could have is mere possession: we have a good in the same way an animal has its kill, until someone stronger comes along and takes it from us.
The gifts we have received from the pre-existing civil order come with an obligation, and that is that we do our part to support it, by, for instance, paying taxes. As Clint Eastwood noted in El Dorado, failing to pay your taxes is the same thing as stealing. And if you steal, you shouldn't be surprised if men with guns get involved in your life.