I am glad we got to your real answer. Which appears to be "we can't do that, we need the ultra-wealthy to spend money on us." One of the lessons of the pandemic is your economy is actually better serviced by many people spending money than by 1 person hoarding it.

You can make the cost of doing business in your country that they have to be worth less than your cap.

But the effect of this is actually that we would realize we don't need the ultra-wealthy. We would likely do better because it would be a significant downward pressure on inequality in the countries that enforced it, and more people would have opportunities to make it big.