Creating a sales cutoff for classes allows you to stop sales in the hours or days leading up to the start time. At the time of this writing, its possible to do this from two places: the template and the individual class. It is not currently possible to do this at the class series level.
To make these changes, click Classes > Schedule > choose a class > then edit either the template or individual class.
Individual class path
In either scenario, you'll find the Sales cut-off menu under 'More settings' at the bottom of the page.
Set a negative number of minutes to allow booking after class begins. (ie. -10 would allow booking 10 minutes after the start time).
For day long cutoffs, use the following: 1440 = 24 hours, 2880 = 48 hours, 4320 = 72 hours.