For a scheduled class, go to the service in your 'Diary' and click the red rubbish bin icon.
This will delete the session and notify all participants of the cancellation.
You can also close the event to prevent further bookings for the session, however, bookings already made will be retained. You must reach out to customers personally to inform them of the cancelled session.
Alternatively, go to the 'Staff & Resources' section in 'Setup' to go to the individual staff member.
In the staff hours tab, use the block out calendar to block dates that the staff is unavailable.
If there are active bookings in a scheduled session for the blocked out day, make sure to reach out to customers to inform them of the cancelled session.
Setup > Staff & resources > Staff Member > Hours