Hi Omar,
I have a bit the same situation and I manage that through pricing. Single rooms are priced for 1 or 2 people, while the whole appartment is significantly more expensive (the price of 6 people). People are welcome to rent the whole apartment as fewer than 6 people, as long as they pay the price.
I don't think you can move people once they are booked, but you can politely point people in the direction of your other listings, for example by telling them it is cheaper and more suited for the number of people.