This only works if you're nursing lying down, but maybe if you do it enough, it might help. When DS is getting too touchy-feely with the other breast, I nurse him side-lying from the top breast, so that the bottom breast is kind of facing into the mattress and is inaccessible for twiddling. I guess I'm lucky, because once he gets a let-down, he becomes much more gentle with the other breast. So I nurse him from the top breast first, to get the let-down without being mauled in the process, and then when I nurse him from the bottom breast afterward, he's not so grabby.