@Wendy1071 This appears to be an Inquiry, not a Request. And yes, stupidly, guests have to enter dates to ask a question. You do not need to decline Inquiries, all you need to do is message back within 24 hours.
If it's a Request, try to get the guest to withdraw it, so you won't have to decline. "As is stated on our listing, we do not accept long term bookings. I would suggest you withdraw this request asap, so you will be free to look for a listing which meets your needs.
Since it seems you are getting a lot of these, I would put a sentence at the beginning of your listing descrption, "PLEASE NOTE: We only cater to those looking for short term rentals. We do not accept long term reservations."