[nemothekid]

>Given that most billionaires have their billions as imaginary ownership of gigantic corporations, how exactly would someone steal their shares from them such that government needs to enforce their property rights?

You have it backwards. "imaginary ownership of gigantic corporations" doesn't exist without government. The government doesn't "protect" Zuckerbergs shares, the government is the vehicle that gives Zuckerbergs shares value. Without the government Zuckerberg's billions is worthless.

In this fairytale world where Zuckerberg is somehow made a persona non grata, then all his shares would become worthless as he wouldn't be able to sell them, nor would he be able to enforce Facebook (the entity) to do anything on his behalf.