I used to work at a shelter and fostered and eventually adopted a 27 pound Tuxie. He had been adopted and returned 3 times due to his "Love Biting" a bit to hard before I took him home to see what was up with him. Some kitties just get over stimulated when they love their human and many mammals tend to nip or bite or like human babies put things in their mouths.

If your kitty bits too hard, its usually best not to react too much, a little tap on the nose, and the verbal use of the same words each time such as "OUCH THAT'S TOO HARD" or some similar thing in a firm but not angry voice will get the point across. Cat are very smart and understand a lot of what we say to them if you use the same words or phrases each time for things.

One other thing to remember that's VERY important is that while your cat is on your lap AVOID the full head to tail stroking when you pet them, as this tends to over stimulate them and their affection. Try just rubbing or massaging the back of kitties neck gently. This worked for my Tuxie within about a week he stopped biting. He's been the best lover boy since and now gives endless head butts and noses (nose to nose rubs) now, and I've had the pleasure of being his human for 9 years now, he's 15 years old and plays with his brothers and sister like a kitten. He's totally a joy to have around.