Then you're going about it the wrong way.
The main thing having billions of dollars gets you is the ability to choose who runs a large corporation. If you take away the money but not the corporation, all you're doing is transferring the power to run that corporation to some Wall St assholes instead of the founders. The corporation is still just as big and whoever is put in charge of it still has just as much power.
The thing that actually concentrates wealth is large corporations. The solution, then, isn't taxes, it's antitrust and fighting against regulatory capture. It's to make corporations smaller and more numerous, so they each have less power.
Which would not only solve the actual problem, it would also solve the nominal problem of billionaires having too much money, because the way you get billionaires is by having corporations that are too big. It causes the people who got in when they were small to have "too much money". Don't let them get that big and that doesn't happen anymore.
The point to taxing billionaires is not that "the government needs their money to spend". The government will spend the money anyway regardless of whether it has it. The point to taxing billionaires is to even things out and nudge society to be more equal counter to market forces continuously concentrating wealth. I'd fully support a tax on billionaires where the money just gets burned in a furnace.