Granddaughter bites when she's frustrated. Any ideas to stop?

My 18 month old granddaughter gets frustrated when she doesn't get her way and will chomp down on any piece of flesh near her mouth. She even has bitten any other object near her to vent. Any ideas to get her to stop biting before daycare starts in August? We've told her NO and that it hurts when she bites but I know that she doesn't understand. I don't want to use any kind of physical discipline.