A ban would garner no revenue.  A Tobin Tax would pay for itself many fold over.  Can you imagine the regulatory body that would have to be created in order to enforce this?  It would be much easier and take a smaller bureaucratic organization to simply tax them at a paltry 1%.  And a 1% Tobin Tax would effectively ban HFT as if they even HFT for 1 second (as they are presently trading per-milisecond) they wouldn't have much left would they?