Ok, first, having a listing suitable for 6 and attracting smaller groups will inevitably end up having bookings for 2, and 6 showing up. Especially if you price it on a per pax per night basis.
We have this same issue, but it's seasonal, rather than ongoing. This is how we've solved it:
In the summer, for us, it's a veritable guarantee that the entire Villa will be booked solid with 6-8 pax (our capacity is 8 pax). So, from April - October, it's flat rate pricing. One price for up to 8 pax, all bedrooms prepared and available, no matter how many pax booked. If we priced it per person, then we'd be booked all through the high season for 2 pax at 25% of the high season rate. We'd surely lose 75% of our revenue.
In the off season, the main market is for 2-4 pax. Pricing it flat rate for 8 pax, with all bedrooms available would price us out of the market. More people cost more. More laundry, more heat, more cleaning, the more people, the later they stay up, using more electricity, heating, etc...
So, for the off season, we price it on a per person per night basis. Starting with a somewhat premium price for the first 2, and a relatively small extra per pax/night for each additional person above that.
The base rate includes one bedroom per 2 persons. 3-4 persons get another bedroom, 5-6 get another, and 7-8 get another. We lock unbooked bedrooms.
Now, if in the off season, we priced it at a flat rate attractive to say 2-4 pax, we'd inevitably get bookings with 8 showing up, because it would be just too cheap to resist. And we would bear the costs of that.
The caveat, however, is that Airbnb has no facility to change the pricing structure on a seasonal basis, so we have to have 2 listings. One for high season, one for low season, block the low season on the high season listing, block the high season on the low season listing.
Ok, simple enough, until you try to sync another platform with these two listings, because the other platform will be eternally blocked. So, we have to manually block Airbnb bookings on other platforms, which requires extreme vigilance, or else you can easily end up with double bookings, and that's not good. It's just another layer of work.
Nothing's easy 😳