Like others have stated, the prop nut must be very tight. There is another possibility(not certain if others have mentioned it yet): The engine comes with a hardened washer that is SUPPOSED to be under the prop nut and against the prop. If that hardened washer is installed on the other side of the prop (installed first, then the prop, then the nut) your problem will occur. I have seen this at the field a few times. The surface against which the back of the prop must bear against is very rough (a crosshatched surface like some metal files) and it is this roughness that digs into the prop and causes it to turn with the engine. This explanation may be difficult to follow, but it contains the information you need.