As Diane Coyle points out, these transactions cannot be ascribed to any particular country but take place in cyberspace. Any taxation would then have to be levied internationally and be universally accepted. The obvious body for this is the United Nations - which could be authorised by an agreement of all the member states to impose a levy on all short-term currency transactions of a speculative nature.
Such a measure could provide the UN with an independent source of funds for its peace-keeping and other activities as well as providing a greater degree of stability for national currencies.
Crawley Down, Sussex