People should not be allowed to accumulate capital beyond $X, yes. What natural law means they should? Society created the conditions for that person to be so successful; in fact, the person only had the minor part in that success. Once you reach $X, you get a certificate saying you won at life and society is really grateful, and society gets the rest of the rewards while they dedicate their life to philanthropy or torturing kittens or whatever it is they do as a hobby.