Evil
Depends where the throttle is stuck, what I mean is which side of the correlator linkages. As you may know the correlater on a 300 will try and open / close the throttle in relation to where the collective is. So if stuck at the hand grip then the correlator will look after rpm. More likely for the linkages to go at the injector itself.
I would suggest that if the throttle is stuck at full open as opposed to the setting for whatever pitch the blades / Map you are flying at then you have a major problem. The throttle is going to want to accelerate the rotor system, it maybe that lowering the lever may activate the correlator to close the throttle, if not then the rrpm is going to go off the roof as you decrease MR drag.
To contain NR you would have to lift the lever, bear in mind you can't enter autorotation as the engine speed is higher than the rrpm so freewheel won't come into play. Your only real option is to pull the mixture which is easier to do then the fuel shut off valve. By gradually pulling the mixture the fuel will go lean increasing engine rpm until the lack of fuel starves the engine. Now can you restrict the fuel flow to slow the engine, I doubt it as if you follow the last check of the day on shutdown in a 300, as you pull the mixture at about an inch out the mixture goes lean, hence the 25 to 100 rpm rise, pull out another fraction and the engine will die ! I would imagine pulling the fuel shut off valve will have the same effect.