Or maybe you can convince them to give away most of their fortunes (still keeping enough to live the rest of their lives in luxury, of course, because what's a few hundred million to these people?), when being a billionaire is no longer allowed.
Like, allow anyone to make up to $999,999,999 after-tax, and anything after that (whether capital gains or income) is taxed at 100%. It's that simple. You only need to "take care" of the ones who try to stop it or reverse it.
Imprison the ones who try to circumvent it with offshore banking, webs of trustees, money laundering, or other loopholes that are only available to the wealthy (like a collection of priceless paintings that are used as a bank account, or yet another personal yacht written off as a business expense).