I had the same problem and this was the reply from Airbnb staff...
Hello Samuel,
My name is Michael with the Airbnb Trip Experience Team, and I wanted to reach out to you today regarding your reservations due to Instant Booking.
When you sign up for Airbnb and activate your listing, guests will be able to book your space without needing to request approval. This feature can’t be turned off, but you have the option to require all guests to meet additional requirements before they’re able to instantly book your space. This gives you more control over who can stay in your listing.
Here are a few ways to control who gets to book instantly:
1. Set your reservation preferences
You can use reservation preferences to control when and for how long guests can book your space. You can also block days before and between reservations to give you time to prepare. Read more about how to set your reservation preferences here:https://www.airbnb.com/help/article/1210/
2. Set guest requirements.
All guests will have to provide Airbnb with certain information to book. You can also decide whether you’d like all your guests to meet additional requirements. Read more about guest requirements here: https://www.airbnb.com/help/article/1170/
3. Set House Rules.
In addition to meeting guest requirements, you can set House Rules that guests must agree to before booking. You’ll be able to cancel penalty-free if a guest breaks your House Rules. Read more about setting House Rules here:https://www.airbnb.com/help/article/472/
I hope this helps! Please don't hesitate to let me know if there's anything else I can help you with.
Regards,
Michael C
www.airbnb.com/help