> The only way to really combat it is for different
    > countries to collude and perhaps sanction tax havens.
    > Even then, I doubt they will be able to raise much tax.

I wonder if the US (and probably only the US has the clout, but perhaps US + EU) that any company doing business there has to show they had paid corporate tax on all profit worldwide of up to 20%. Doesn't matter who they paid it to, but if they paid any less than that, they owe the remainder to the IRS, as a cost of doing business in the US. Could that work?