This may sound dumb but why not use a bit of restraint on the throttle finger? Other than that it may cost a bit to replace the radio and or batteries (like using a 4 or 5 cell pack). Gearing down would help slow the top end but the acceleration would be quicker and easier to spin out. If all else fails tape a small block behind the throttle to limit it's travel thereby slowing the car.

I don't think there are EPAs on the RTR Tiger/Associated radio only trim pots and reverse switches.