They are trying to tell you they don't like that- I'm not sure if there's a way to get them to stop. Because they are prey animals, they have the instincts of prey animals, and the neck and underside are very vulnerable places. Nothing in the wild tries to get at your throat or underside because they want to pat you! And I've noticed over the years that most animals hate having their feet touched or held. My pigs will let me stroke the underside of their feet lightly if they are laying on my lap stretched out with their feet out the back, but woe is me if I try to touch their front feet ever! They probably think I'm coming for their nails to clip them, which they all hate!