@Verity33 It doesn't sound like there's any confusion happening here. These guests are playing you.
They are not entitled to any money back, and you should not refund. There's probably a bad review coming your way regardless, so don't give them anything back.
In reality, they have violated your house rules by bringing an extra guest and refusing to pay, so you have grounds to kick them out, in which case they'd get no refund.
Next time, don't call AirBnB to ask what to do. You have rules for your property, and guests have agreed to abide by them when they book. Get AirBnB involved when you are preparing to kick them out. That's it. AirBnB is a booking platform only, and does not have your best interests at heart.