A sufficiently progressive wealth tax does the trick. If wealth in excess of a billion is taxed 50% annually, multi-billionaires won't remain so for long.

There are only two operational challenges, but if the political will is there, they really aren't rocket science. One is having enough auditors to enforce the tax. The second is liquidity, which can be solved by allowing the tax to be paid in kind, i.e. by allowing it to be paid by transferring ownership to a social wealth fund instead of cash.