First, I would like to introduce myself, I am new to the forums. I also saw a lot about pwn or p0wn which I was not really following, but I think the original question was akin to how can a person be owned. I will answer this question only from my perspective as a Dom/Master and tell you how it has manifested itself in some relationships I have been in/currently am engaged in.
1. There is no legal way to own a person, except in the sense of something like a marriage contract where both people make agreements to each other and state they will do these things for as long as they both may live etc. However this is still something that can be ended by either of the parties beginning a process to terminate the marriage, and also it is implied in marriage at least in current day that both parties enter into it equally.
2. In BDSM at least in how I have experienced it, there can be ownership, albiet it voluntary at least in theory. I have "owned" 5 women in my life as a Master, in the cases of these relationships I drafted a contract that detailed the conditions of the relationship and the sub agreed to it, and it included consequences for actions outside of the contract. (both for Master and slave/sub). Typically these consequences included some type of punishment administered by the Master to the slave that were within the control of myself as Master. Of course the bottom line is that the woman could decide at anytime that she wanted no longer to be owned and would be released. Unlike in forced slavery where a slave's only option to end servitude was to run/escape. The other aspect of this is that even though the Master may punish the woman, and in many cases control certain aspects of her comings and goings he must also be nurturing and supportive of her for a true ownership situation to last.
I have also been in several BDSM relationships where there was no ownership, in my opinion what seperated it was the contract, which is actually more of a covenant than a contract, it is something agreed upon by both parties and the sub is trusting the Dom at an even higher level.
Also, do not try to get such a covenant/contract notorized etc. There have been some Einsteins out there that have taken such a step, obviously the contract is not a legal document, but a voluntary covenant.

2.