One more thing...have you checked the programming for "Throttle Hold" in your TX?

I just put together a spare 450 that I bought as a parts heli because xHeli had them on sale. Anyway...I put in a brand new Alpha 400 I had as a spare as well (I bought 2 when I built my main 450 so I would have a backup motor) and on my main 450 I have Throttle hold at -5, but this new build...the motor still turns at -5 so I had to go to -10 in order to get it to stop. These are 2 identical motors!

So...if you use throttle hold...this could be something to look at...or not. Just trying to throw you a bone. Im still very much a n00b so take my advice for what its worth. lol