[mikeash]

You don't need to define HFT, nor do you need to play games with values. Set the tax at a raw percentage of the transaction's value. This percentage can be ludicrously low, like one hundredth of one percent, and still achieve the desired outcome. Such a tax will have no appreciable effect on "normal" trading, but would seriously diminish HFT.

I have no idea whether this would actually be a good thing or not, though.