@Brandee6 Are these requests to book, or just inquiries? If they are just inquiries, you don't need to accept or decline: you can just send them a message stating that you don't accept third-party bookings, but if their friends want to contact you directly, you will consider their booking request.
If you don't have something in your rules about it already, maybe add it right at the top of your rule section.
You are definitely doing the right thing by not accepting third-party bookings. Airbnb doesn't allow them, and you won't be covered if something bad happens. Stick to your guns!