How to deal with “Scrooge McDucks” in my fixed-currency-amount game?

As an economist, I have to tell you that fixed-currency-amount systems (such as the gold standard and bitcoin and other fixed-amount cryptocurrency systems) are derided for some very good reasons, including those expressed here: it’s possible for few agents to essentially hoard all the currency the economy needs to run.

My best suggestion is instituting a heavy negative tax on funds in excess of an arbitrary large amount that remain un-moved for a certain reasonable lapse of time. The taxes would then be re-distributed equally to all other players’ accounts.