We need to wipe them from existence.
Let every nation have a maximum wealth cap. 1000x the median national household income. Anything else is taxed at 100%. This cap in the US would be about $80 million today.
But if you're somehow in flagrant violation of this limit? If your wealth is the equivalent of over one billion 2025 dollars? Unless you quickly earn a multi-billion fortune and immediate give it away or spend it, having a net worth over $1 billion will be a felony. It will be a capital offense. Oh you're secretly billionaire, having hid your vast illegal fortune from the IRS? You are guilty of trying to amass a geopolitically-significant level of wealth. We're going to treat this as seriously as we would someone who attempts to build their own personal nuclear weapons. Really, you are guilty of a form of treason. Except instead of aiding a foreign nation, you were trying to become a threat to your nation yourself. And the traditional penalty for treason will be applied.
This is the world we could have. We could, if we wanted it badly enough, simply make being a billionaire a capital offense. Force them to give away or spend most everything they have. Either way, given or spent, wealth is distributed enough so that no individual can threaten the nation through their own personal wealth.
It is time we wiped billionaires off the face of the Earth. No individual should ever be allowed to become so wealthy that they become a threat to nation states.