Unfortunately the BDC review system doesn't leave you much recourse. Incidentally, I had a very similar issue just this week. Someone booked our 'Small Double' room, for two guests. They showed up with a family of FOUR, and then left a review saying we changed their room without telling them and that they booked a room with two double beds. We don't even have a room with two double beds to book!
I charge a non-refundable deposit at booking, so I always have a validated credit card to charge in case of no-shows, and fraudulent bookings are detected immediately. When I've used booking.com payments I had issues with them refunding guests who should not have been refunded, refusing to refund guests when we were willing...if it works for you great, but I much prefer to handle payments myself.
I disagree with Blagoj, and suggest taking your own payments. It's much better to be in control of your own finances.
Go to your extranet -> rates & availability -> set restrictions.
As Blagoje said, bdc doesn't need your permission to book guests, and once booked you are obligated to accomodate them or find them alternate accommodation (or you can try requesting cancellation). Putting the property live without your permission seems fishy, though. I'd get in touch with your bdc rep.
Always make sure you have a valid credit card! I advise getting a POS terminal as pibomarco suggests, and process your payments/deposits in advance. A happy side effect of all this, is that people are far less likely to cancel if you've already charged their credit card.
I also have this issue, but I have not spoken to booking.com as it's generally a frustrating and ultimately fruitless endeavour. I would be interested to hear if anyone has.