I sent a message to the guest that my insurance only covers for registered number of guests and asked for the unregistered guests to leave. The guest didn’t respond. What do other Airbnb hosts recommend doing?
Make it clear that any unauthorised guests will result in immediate eviction, put it front and centre in your house rules and CS will support you. You'll get paid for the full booking with no risk of retaliatory feedback. I can confirm this as I've cancelled more than one booking for those reasons.
I've just added a massive extra guest fee because I really don't want more than 5, but I've had people being more. At the end of the day I only put out towels and bedding for the number of guests that are on the reservation. We mention in our listing that if extra people stay overnight, the extra guest fee will be incurred.