You can do this from the roster view.
Go to the Upcoming tab in the Sawyer menu
Click on the individual class to open up the roster view
Click on Roster Actions to the right of the Message button, below the +New Order button
Select "Cancel Instance" from the dropdown
This feature is designed for providers who cancel just one class out of a series--if, for example, there is a snow day--without preventing customers from signing up for the series as a whole.
Canceling a class does not communicate to customers that it has been canceled and does not refund them. You can send a class-wide email using the Message button and issue refunds to each customer if you so choose.
To remove every class instance for a scheduled activity and prevent customers from signing up, refer to this short Help Center about how to delete a scheduled activity.