Oh, sorry, it's this very simple idea that every trade is subject to a tiny fee (a few basis points). Totally irrelevant if you buy for the long term, but utterly destroys the business model of HFT.

https://en.wikipedia.org/wiki/Tobin_tax

EDIT to add: Appropriately enough, Wikipedia says:

> Another term for these broader tax schemes is Robin Hood tax, due to tax revenues from the (presumably richer) speculator funding general revenue (of whom the primary beneficiaries are less wealthy)