@Tammy your suggestion re tracking repeat or serial 'refund requests' by the same guests is a great one; Airbnb already have all the information to hand so it would be actually really easy to extract this type of data and also very useful. It's great that you sent the feedback to Airbnb and I hope you highlighted that you were also making a suggestion in addition to the feedback.
Re the feedback, you must always leave a genuine feedback in order to warn other hosts; also guest’s will eventually have to get their act together if their feedback continuously reflect issues as they will find it hard to book future accommodations.
If this guest is a serial refund requester and a previous host had left this message then you would probably have declined their request saving you all this trouble.
It defeats the purpose of leaving feedbacks making it a pointless exercise if it does not reflect the true experience the host or guest had; feedback must also never be left in retaliation of a negative feedback. In fact not leaving a feedback that expresses your concerns is as good as Airbnb know serial refund requesters and not sharing the information, in both cases, the guest continues to get away with poor behaviour after all there is no deterrent and only what seems like an incentive to continue in this manner especially of they get the accommodation for cheaper.
I must say you made a big mistake which could have had repercussions for you by allowing additional guests who were not on the booking confirmation to stay in the property. If there were any problems which you had to report to Airbnb, you would not have had their protection because you had more people than stated on the booking confirmation in you your property; therefore you exposed yourself to additional risk and those additional people were not covered by Airbnb protection. This was also an opportunity to cancel this guest’s reservation straight away without any penalties if you were not sure/ happy with him.
Re contact, the best way to make non emergencies contact is via twitter and to be honest it is quicker than phone calls sometimes - click on the link for contact options - Contact Airbnb
Wish you all the best, Ama