@Miriam164 You can message the guest the same as I suggested above, and ask him to withdraw the request. Let him know that you have to either accept or decline within 24 hours (from the time you got the request), and that if you decline, you will recive penalties, so you will accept, but only on the condition that he books for no more than 18 and that if more show up they'll be turned away. And that if you accept it before he withdraws it, he'll have to cancel within 48 hours to get his $ back and wait up to 15 days for a refund..
Or just decline as long as long as you don't already have so many declines that it will hurt you.