@Allison93, "no good deed goes unpunished...." You tried to help them out by letting them check in early, before the cleaners had finished. So if they cleaned too thoroughly, i.e. threw out their food, that shouldn't be your problem! I personally only allow early checkin if the cleaners are finished... First impressions often last.
I think you've been more than generous with these people and I would just let them stay, not refund them (charge them for the extra time they had when you let them in early!!) You may get a negative review but you'll have a chance to comment on it. I would also definitely leave a frank review on their behavior so that other hosts will know what's coming down the pike if they host them. I don't think reimbursing them will prevent a negative review. Also as others have said, how many people are actually there? sounds like more than your maximum, so you may have some grounds for standing firm as far as ABB is concerned, e.g. they booked for x people but then you found x+y were actually staying there.
Be sure to have ALL communications and payment go through the ABB official systems so it's all documented in case of dispute later e.g. they cause damage to get back at you etc.
Good luck- rest assured the guests are not all like this!