I have yet to convince people they cannot tie my shoelaces. Telling my friend I could do it as fast with one hand as she could do it with two hands was not a deterrent. She said "I could tie your shoe at least once so you won't have to." My strategy to handle this more gracefully next time will be to point and say "Can you tie a shoelace that has only one free end?"
P.S. The strategy above works. Able-bodied people no longer get upset when I will not let them help me tie my shoe.