The capital gains tax could be 90%+ on any gains over $10m (and 99% over $100m), so they can still sell out, but it would cap liquid wealth at $100m dollars. However, they would still have a billion dollars of ownership, and the societal power that comes with being a billionaire, as they can leverage those assets to great effect without selling them. (As Elon Musk didn't actually pay $44b for Twitter.)

So I wonder if the government could levy a stock tax, in which every public stock was diluted by 1% per year, with non-voting stock going into a sovereign wealth fund. This would make for a gradual transition of companies from capitalist-owned to public-owned.

Oh I thought that they had that much money in liquid wealth. My bad.