Good questions... I've had the same problem. I was origionally open until 6 when I started care. I then had a group where no one needed me past 5:30 so I re wrote my hours, but didn't feel the need to hand them out as no one needed past then.
I simply don't post hours anymore. I advertise "flexible hours" And my contract is based on hours they need at the time of signing up.
I can't help you with how to deal with it as I simply just sent out my revision when someone came after 5:30. Anxious to see what other's say though.