That's it. It's incumbent upon asset-holders to remain under the limit. It's up to them how close to the line they wish to tread.
No one becomes a billionaire ethically...no one produces that level of value, and absolutely no one needs that much money. And we have evidence that it literally breaks your brain when you attain that level of wealth. It's only possible to attain through exploitation. If you are found to hold a billion dollars in assets, you go to prison and forfeit all of your assets, so it's up to them to manage their assets accordingly. Give it away, pay your workers more, distribute it however you want, but a single person cannot be allowed to control that much capital - full-stop.
If you flee the country, any assets you leave behind are forfeit and seized. You will not be allowed to retain the wealth you accumulated on the on the backs of taxpayers and labourers...it will be forcefully redistributed for the public good.
For any country that does this, extreme wealth disparity will be eliminated. If there are countries that allow billionaires exploit the working class and public infrastructure, then they can go build their fortunes there. Equitable, civilised countries will outlaw it. Billionaires only exist because we allow them to.
Someone with less than a billion will be just fine, and can enjoy a reasonable, moderate amount of wealth in peace.