Search for the Activity
Click on drop down and select 'Select Activity'
Go to Activity Options > Sessions
From the list of Sessions select the Session you wish to cancel
Update the "Public" field on the session to "No, Private."
What this allows you to do is still maintain the session from the administrative end, but it prevents the session from showing on the calendar and/or any public searches (such as the search when claiming credit).
If this session is eventually totally cancelled, you can update the 'Cancelled' flag to Yes.
A cancelled session will be unable to be claimed, and it will show as cancelled in most admin searches and be left out of some reporting unless explicitly included. Cancelled sessions are also excluded from announcement.
IMPORTANT: If you inactivate the session, it is essentially deleted from all reports and searches. if you want to reactivate an inactivated session, go to Sessions > Advanced search > Additional; Set Active = Inactive > Search