Citizens United was one of the worst SCOTUS decisions for democracy. It's not just the billionaires though, politicians have gerrymandered themselves into permanent seats. They pick their voters, completely backwards.
check out some Michael Sandel, this is central to his thesis, and comes from being around all walks of life. We self segregated by wealth and politics, no longer seeing how others live or hearing how they think, IRL
Banning them from politics would be a good start.