Reservation not valid

I do not agree for a reservation for 5 people (2 adults and 3 children). It is indicated well in the conditions that we can welcome 2 children maximum in the apartment this one is too small. I sent several messages (in English and Russian) to the person and a demand(request) of free of charge cancellation by explaining why but I stay without answer from him/her. You can tell me how to cancel this reservation which does not suit to me.
In advance thank you,

Blagoje Acho

Contact b.com and explain your situation. So you say booked apartment is too small for 5 people? How did they manage to booked it. Your setting is 4 persons for that room?