There are a couple of options on how to suspend or pause bookings for a session that you already have on your schedule.
One option is to amend your sessions and change when booking opens and/or closes for that session. Just navigate to the session you want to edit (or the next in a series you wish to update), select Edit this session and scroll down to the Booking opens field and select Set time to set when booking will open for the session(s) by either how many hours or days before the scheduled start time of the session. You can also set a Booking closes time, either setting booking to close at the start or end time of the class, or you can set a specific time period by minutes, hours or days before or after the class. Using these settings you can suspend booking capabilities until a time that you wish.
Alternatively, if you want to completely suspend bookings indefinitely, on the same edit screen you can amend the Booking and listing option to Listing only. With this setting the class will still be listed and visible on your schedule (and any class finder engines you may be linked to on external websites) but there will be no Book button for clients. You can then reverse this change at any time you wish.
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