I guess a follow-up question would be do you think they are being malicious? Almost everyone out there doesn't know how to deal with a stutter so they may be, in their mind, thinking they are dealing with it well but in actuality hurting your feelings. Maybe if you are uncertain of their behavior you could just be straight-up about how you are feeling and how the way they act towards you hurts your feelings. If they are good friends, they will probably understand and try to change the way they are acting.