African, Thai, French scammers

We have a new pastime!

So several Chad gentlemen, currently in France, whos address is oddly the same one in Bangkok, reserving our rooms for weeks at a time with credit card details that fail - oddly that too is the same card!

So tell me, why does BOOKING accept a reservation from a guest using a card which has been proven to not work 3 times in the previous 24 hours?

Our property is becoming un-bookable because of these.