Contact them through AirBnB and tell them you are going to have to change the reservation to 2 guests, that they need to agree to the change or they have to leave because they have broken the house rules.
Always alwasy ask how many people will be staying. Some people don't see the drop down box and think that they are getting the room/studio for two people - just as most hotel rooms are per room rather than per person. It can be an honest mistake - we have had it twice with newbies - or it could be someone trying it on.