Politely declining a booking, whether for a professional service or a vacation rental, requires a "sandwich" approach: gratitude, the refusal, and a well-wish. Start by thanking the person for their interest or for thinking of you. Then, clearly state that you are unable to accept the booking without being overly defensive; phrases like "Unfortunately, I don't have the capacity right now" or "Regrettably, the dates requested are no longer available" work best. You do not need to provide an exhaustive list of reasons, as this can invite negotiation. To leave the door open for future opportunities, you might add, "I hope you find a great alternative for your stay," or "Please do keep me in mind for future projects." Being concise and clear is actually more polite than being vague, as it allows the other person to move on and find another option quickly without wondering if you might change your mind.