At the risk of being accused of child abuse, and losing all credibility with my lactnet friends and family, I will say how I ended that behavior with my son at about 2 months of age.  The first time he did that I reacted with a scream and a yell.  Well, the little mother-sucker thought that was great fun.  I could just see the thought running across his brain, "can I get that reaction again?"  And just as he sought to find out, I bopped him on the top of his head.  He never did it again.  He went on to nurse for about 18 months, never had food or security issues, and 25 years later, we are still very enamored of one another.

Sincerely, Chanita