Classes are only bookable if:
Your classes have forthcoming event dates on them
The event has got spaces available
There are valid ticket types
You can check the status of any of your classes in your timetable by looking at the 'book' icon. It will be red if bookings are switched off, orange if some of the classes are inactive / there's an issue, or green if they are good to go.
Switching Off Bookings
If you've sold out on one specific date, you can simply change the capacity / number of tickets available on that class so that no more tickets are available.
If you want take down an entire series so that bookings can't be taken on any of the dates, e.g. 'Mondays at 10am at St Church Hall', then view it's profile by choosing the class from your timetable.
On the right hand side is a section titled 'Bookings Enabled' - it displays a blue book icon if active. Choose 'No' and the book icon will change to red.