I come from a semi well off family on my father's side and have many well off friends. We all have either trust funds (family trusts) or our parents and grandparents just give us the money outright before they die or we get non-voting shares in our parents holding corporations that own 99% of the company or we get physical gold and silver before they pass.
The list goes on and on. The only people I have ever heard paying real inheritance tax were from the middle class. Rich enough to tax, poor enough to lack a good tax attorney.
There is no real way you can stop it. Even if you prevent parents from giving money to their children (!) then the rich can just turn it into gold chains and give them gifts of jewelry, or turn their wealth into priceless art and then give it to their children, or offshore bank accounts co-owned by father-son.