Your post struck a note with me.
We had a series of wonderful, quiet single guests and noted we enjoyed hosting more when it was just one person. So we changed our listing to say “1 guest” and had the max number of guests set to 1. Well then people started booking for one and said in the booking message (we have IB set on) they’d be bringing someone. So I added as the first rule, 1 Guest Only. People still kept booking for two because I figured out they didn’t see themself as a guest, the person they were bringing was the guest!
I have since given up on 1 guest only because cancelling every other booking that wasn’t a single person was going to be tedious and sour me on hosting. Turning off IB so I screen every guest would be the only way to ensure only one guest and I didn’t want to do that. So now we’re back to accepting 2.
I did have a concern with my listing set at one and allowing the extra guest: I wasn’t sure if they would be covered by the Airbnb Host Guarantee. I know the Guarantee is worthless for small time damages, but it does cover big things like the guest falling and hurting themselves. I don’t want that risk exposure if Airbnb won’t cover them.