What should I do with future "Pledged" Opportunities if a donor cancels a Recurring Donation?

If you are using the Salesforce Nonprofit Success Pack to track Recurring Donations and have set the NPSP Settings to create "Pledged" Opportunities for future donations, if a donor chooses to cancel their recurring donation, you can change the Open Ended Status on their Recurring Donation record to "Closed." Per Salesforce, once you do so:

"All open child donations will be changed to a Closed Lost stage by default, or whatever you’ve specified in your Recurring Donations settings."

If you wish, you may also wish to delete those "Closed Lost" Opportunities, should it better fit your reporting purposes.