I find it hard to believe in the age of tracking that we live in now that banks and consumer transactions that it wouldn't be hard to implement proper tax evasion tracking across borders. This is purely an issue of there being no political will to do this.
It would be even easier to just cut off the tax havens from the global economy - make it illegal for banks to conduct transactions with those banks / branches located in those havens, strand those assets from the global economy destroying the value of those bank accounts.