My son started that a month or two ago... he's 7 mths now. He has gotten me more than my daughter ever even tried. She stopped after 3 attempts- 1 without teeth and twice with teeth.
Since baby doesn't understand the words, a scream followed by a firm "no" is fine. It's all in your tone- hurt followed by firmness- and body language. At that point the feeding is over, and I put him down on the floor with toys.
Watch him like a hawk after the steady sucking has stopped. This is when we are danger from our son's bright ideas. He may even be extra cute and pull off to smile at you or make lovely eye contact. Get your nipple outta there! Sit him up for burping and play with him if he hasn't bitten you. (Since he's older and done this a few times, keep one eye open if you like to nurse in bed in the wee hours. Once the steady suck-suck-swallow has stopped you need to become conscious to protect your girls.)
If pulling him close doesn't stun him enough to let go, get your finger in his mouth so you can wedge his jaws open.
Also if you feel his teeth while nursing, feel free to break the suction with your finger and have him latch on again. If he's really nursing his tongue should be shielding your nipple from his teeth... so he's either slipping off or thinking about using you as a teething toy. You could also give him a Teething table (I've used Hyland's brand available at most stores) before trying to nurse again. (I say one or you'll have that stuff all over your areola.)
Hope that helps. It has kept my nipples safe from my biter.