@Igor34
A bit late now, but contact them thru the ABB messaging system saying they used an extra room and that you will have to charge them for it. Make a change to the booking and ask them accept or that you will have to clear the extra room. Of course the guest has 24h to officially accept, however having communicated like this makes it official and on record in case you need to contact ABB later.
Do you check visitors in personally? It's advisable to be clear then about what they booked and what not.
Also with your hostel-type accommodations you should have locks on the doors, both for protection of guests' belongings as well as to prevent such room expansions, since it's an issue waiting to happen as you've now seen.