There are so many tax havens across the world. And tax evasion is a serious issue as the working class are forced to pay taxes but the rich escape. International organizations like UN have the power to impose sanctions and restrictions, but why aren't they imposed on tax havens ?
The UN can only do what its member countries want; it can't act independently. There are also the 5 "veto powers", who can individually kill anything they don't like.
Tax havens could be frozen out of the world economic system quite easily. Central banks and national regulators could simply insist that their national banks do no business with banks in tax havens.
An important method of laundering money is via a shell corporation registered in the USA. Delaware is often mentioned in this context, as it is an attractive place to incorporate for a number of reasons, and it doesn't require a company to declare who "really" owns it. Offshore bank accounts are only one part of the laundering and evasion ecosystem.
The broader ecosystem is an important factor here; money laundering is rather like the Internet: it's very good at finding routes around censorship.