@Michael2523 I also host Solo travelers only and don't think I'd want to host couples, even if I had the space for that. Solo guests tend to be self-sufficient, well-traveled, friendly, and good communicators. My guest room has a single bed and not even the space for a double without it feeling cramped.
"For Solo Traveler" is also part of my listing heading. While I've had a few inquiries like "So is it just for one person?", they have come in the form of Inquiries, so I've not had to decline, I just message back. A return message on an Inquiry is all that's needed, you don't have to Preapprove or Decline, you won't be dinged on Acceptance rate. On a booking request, you do have to Approve or Decline within 24 hours, but you can also just message back explaining that it's just for 1 guest at a time, and ask them to withdraw the request, otherwise you'll have to Approve it for one person, funds will be drawn from their account, and they will then have to cancel. If they don't respond within the 24 hours, Approve the Request with a message that hosts receive penalties for Declining, thus you are approving the booking, but for 1 guest only, as per your listing settings.
If you want to stick to one guest only, how about replacing that bed with a single? That should make it a little more obvious, if they don't see a photo of a bed that looks perfect for 2.