How to remove an "Individually Scheduled" (ISO) Opportunity from search
Unlike Date and Time Specific occurrences, there is only one occurrence record used to track all connections to "Individually Scheduled" opportunities. So we don't cancel the occurrence. We just want to prevent further signups and remove it from search.
You don't want to cancel an ISO occurrence, as it would affect all connections that have already been confirmed and scheduled. You would also lose the hours you previously logged for connections to this opportunity.
1) Set the end date of the occurrence to yesterday. This will prevent anyone else from signing up, and you'll also have a record of what time period people were able to schedule themselves for. It will remove the opportunity from search in 10-15 minutes.
2) If you like, you can set the occurrence and/or the Volunteer Opportunity to inactive. This too will remove the opportunity from search.
Method 1 above has the advantage of letting you know the time period (start date - end date) that the opportunity was available for people to express interest in.