+1! You can try to apply morals to companies in an internationally competitive environment all you want, but it ain't gonna happen. The only way to ensure they can't play governments against each other is to effectively make a cartel. How you'd get over the collective action problem and prevent countries from defecting is beyond me though, would probably need something like the OECD to pass a resolution.

Most current tax havens are UK dependencies, so such an agreement would have a significant impact. Additionally, individual countries could do something like what the US does with its citizens - if you've paid a sensible amount of tax abroad, great; if not, pay the difference here. You can locate wherever you want, but relocating to save on taxes becomes moot.