[LunarBerry]

This may or may not help but with my furbaby I reduced her harder biting by making gestures with my hand like a mouth and I even did facial mouthing gestures/expressions at the same time (while placing my face close to my hands) to give her an idea what I was mimicking or give the feel that I was actually mouthing her. I would almost always lightly "mouth" her with my hands, if she did it a bit harder I did do the yelp and then whimpered some and/or may ball my hand up and have the back of my hands face towards her, not giving her access to my fingers and she always have stopped and tried to sniff around my hand and looked either confused or concerned, but she eventually realizes what is going out and calms down immediately. If it got persistent, I may "mouth" her a tad firmer to get the point across. Otherwise I always did it lightly and she has been very good about not biting hard.
The only times she bites me hard/firm was for two reasons. One is when I'm doing something she really doesn't like at all.. even then it's not really hard but she does try to get her point across at times. If it was with a stranger or someone she's not as familiar or comfortable with she'll get real stern or even spiteful about it. And the other times, it's not intentional though, is when I have a treat that she's really wanting or gets excited to eat and she over chomps the boundaries :P