My 17 month old daughter just started grinding her teeth. What can I do to make her stop?

It is normal. No need to worry. Discuss it with your dentist in her next scheduled dental check up but nothing to worry about in the mean time.
Not much. In three decades I have seen teas/tablets etc offered as cures but found nothing that really works. Some kids do this for a while & stop on their own. Dentists can create a metalic crown for back teeth that prevents the front incisors from wearing down but that is a bit drastic for most parents. Talk with your dentist/kids dentist & see if they can offer specific suggestions for your child.
Tincture of time. Kids grind teeth for 2 reasons. 1. Their jaws grow in different spots on different days, and they are grinding in their occlusion, and 2. They back up the "computer" at night. They learn so much each day that they need time while asleep to process it all. Usually stops on its own before permanent teeth start coming in. Give it time.