By short circuiting both the field and the load connections of the alternator, either with a bit of (thick!) wire or, perhaps even more prudent, with a low resistance, a car bulb or so
I was tempted to say that, but didn't want to push my luck! I have experimented quite a bit with car alternators (which are basically the same technology). I once had to jump start my car after finding the battery had failed completely open circuit whilst driving - predictably a week after the warranty had expired
. I deliberately left the side lights on, to provide a resistive load, and no damage ensued on the return journey. Diodes have both a forward and a reverse maximum voltage, and exceeding this is asking for trouble. With
no load on the output even a small amount of field magnetism can produce surprisingly high stator voltages, and consequently the risk of damaging the diode pack.