Only offer instant booking if you keep your calendar up to date. Are you also taking bookings through another listing. If so you will need to sync your calendars.
As you see BNB takes host cancellation seriously (look at their help centre for more information on penalties) and so do guests it will put them off making a booking with you.
Sounds like you would be better removing it from your listing. Just go into your listing and then bookings and untick it. BNB will encourage you to stay with instant book but keep going through the steps and eventually it will change.
It sounds like you would also benefit from reading the community guides on these forums - great for new hosts.