Outright banning using collateral for loans wouldn't work, even us plebs can do that and it has uses, like taking out a loan against your house. And if it's not stocks, its something else they have that's worth a lot or will loophole around into. They just need to tax collateralized loans or ban it after a certain amount.

edit: and if you get to the point that its taxed (if its at a threshold) then you got enough money to pay someone to figure it out and manage it.