At about 10 months my daughter pinched my neck whenever she was nursing. I looked like I had some weird skin condition... my neck was always red and blotchy. One thing I tried that worked pretty well was to wear a turtleneck so she couldn't get to that part of my neck. Since it's still pretty cold out, can you wear an extra layer over your arms?
My daughter did outgrow this phase on her own, although it took more than a month. Since she was only 10 months old I did not try pinching or slapping her to make her stop.
Now she's 13 months old and has teeth, so I need to worry about her biting. She doesn't make a habit of it, but whenever she bites I exaggerate my reaction, yell "OUCH" and jerk away from her. (Often this is an honest reaction because it HURTS!) This seems to be enough to upset her, and hopefully that will reduce the instances of biting.