I don't think this curse/blessing is being fooled by legal loopholes like trusts. If you can control the money and spend it however you like, it's yours, regardless of what name is legally attached to it. So I think the wealthy heir would just immediately die. I assume the magic or divine will can recognize the True Owner of an asset.

I assume this also prevents wealth-hoarding by proxy. Imagine a mob boss who wants to safely own a fortune in billions. He finds people with a lot to lose. Good jobs. Family. Nice house. Etc. Respectable upper middle class people with a lot to lose, but themselves nowhere near a billion dollar limit. The mobster offers the following deal-you-can't refuse:

I am going to wire you ten million dollars, plus some for taxes. You will report some $10-15 million in income as taxes next year. After paying the taxes on this, you will be left with $10 million. You are to set this money in a bank account and never touch it for any reason. I may access this money at any time by requesting a wire transfer from you. Your new accountant we're hiring for you will take care of it. In exchange, you may withdraw from the account a fee equal to half of the interest earned from it. You may also use this portion to cover the taxes from the interest. You will be periodically audited. Do this and you gain income without risk or effort. Don't do this and we torture your family to death in front of you, kill you, then burn your house down around you. You have five minutes to decide. (Goons walk in holding knives and gas cans.)

We're not talking law here. We're talking magic or DIVINE WILL. I think in the above situation, the magic would be able to recognize that the money really belongs to the mobster. Legally it's in the name of the patsy, but in all practical terms, it's still the mobster's. Only he has any authority to spend it. If the actual legal owner of it touches it, he condemns himself and his whole family to a gruesome death. No. I think any proper magic will simply recognize the True Owner of any asset, in the way that magic famously often recognizes True Names. I think any wannabe Light Yagami is just going to get exploded.

The heirs of billionaires need not actually condemn themselves to death though. You can legally refuse portions of an estate. Different jurisdictions have different ways of handling unallocated assets, but there would likely be a long chain of heirs listed in a will. Each would take an amount safely below the boom limit and decline the rest. If the list of heirs in the will runs out, I suppose state law takes over on who gets the rest. Some may just end up in government coffers.