What does work is consumer pressure and the consequences of bad publicity for companies who cheat us of their tax contributions. Every little bit of pressure helps, so let's keep on crowd-sourcing our concerns - although I do agree that we've probably exhausted this thread as a channel for these issues.
In terms of what governments could actually do a significant chunk of tax havens are ultimately British. Cayman islands, Gibraltar, Channel islands, Isle of man, British Virgin Islands, Turks and Caicos Islands, Bermuda. The parliament of the United kingdom has the ability to shut them down tomorrow (its not going to because the financial heavy lifting is traditionally done through the city of London but it has the legal power to do so).
Of the rest we are for
the most part talking near micronations that could be trivially
terminated by neighboring governments.