Argh, DD went through that stage as well and it's so hard! She did eventually grow out of it, but it's a tough stage. Be firm, consistent with the "No, that hurts!" but I found it also helped to then either put her down if I was carrying/cuddling her or if at home and playing with her I'd stop playing and walk away, find something else to do.

It kind of says that you don't think it's funny without making a big deal out of it or giving it a payoff of lots of attention, if that makes sense?