Once they reach $100bn, confiscate all their wealth and make them start again with a market stall and a small loan (say $1M). Call it Billionaire New Game Plus.
My better solution: On January 1st of each year identify the wealthiest billionaire and just liquidate that one individual. Then everyone can still play the game where we all try to gain more wealth, but with a little meta-game included where you don't want to get too rich.