If they call again for help, be honest and tell grandma that you will be happy to stay with grandpa, but that you will not take care of the child as he did not listen to you last time and they must come home directly after the appointment. If she tries to say he will be good this time, etc, etc, or they need to run errands, stay firm and repeat "I will stay with grandpa but child cannot stay. Also, you will need to return home as soon as your appointment is over, no shopping, as I have other commitments. If that won't work for you, you will have to find someone else."
Repeat, repeat, repeat.
If you get there and they try to leave child with you, gather up your things and prepare to leave. Do so if you need to. Also remind them that they need to come right home. If they offer to have someone else come in after xxx hours, take them up on it. Get the number of the person and contact them yourself to make sure it is really going to happen.
It is great to help out your grandparents, but that doesn't mean that you need to be taken advantage of or watch a child who will not listen and creates stress for you.