If the Cancel button is greyed out, the booking is past the point where you can leave it yourself. This protects the other players: leaving late could mean the session is cancelled for everyone. Contact the club if you need to leave, or find a substitute for a match.
Matches
You can leave a public or private match until the club's cut-off before the start. The club sets how many hours. After that, Cancel is greyed out, however many players have joined.
In a match where players pay their part, you can tap Find a substitute instead. Another player takes your spot and you get a full refund once they pay. See "How do I find a substitute for my match?".
Activities
You can leave an activity right up to the start time.
The exception: if the activity is already below its minimum number of players and inside the club's automatic cancellation window, Cancel is greyed out.
Courses and camps
To leave a course, cancel from the first session, before it starts. This removes you from every session.
Once the first session has started, you can't leave the course in the app. Contact the club.
Other bookings
Court bookings, private lessons and league matches can be cancelled until they start.
Nothing can be cancelled in the app after it has started.
Bespoke bookings made by the club can only be cancelled by the club.
Who can help
Your club can remove you from a booking and decide on any refund. Nettla can't cancel bookings or give refunds. See "How do I contact my club?".