A: Well that’s up to you and your network. You can’t delete a campaign once people have donated money to it, otherwise that money would sit in limbo and not make it to any good home. If your supporters would like a refund, you can contact the Charity via the Contact Organiser button on their profile page, and ask them to process a refund through Stripe. But you may find your supporters are a pretty good bunch and are happy for their money to still go to the good cause.
Alternatively, you may wish to edit your campaign and create a new fundraiser for the funds to benefit instead. Note you can’t change the charity benefitted, but you can change the other details of what you are doing. Sprained your ankle so can’t run the marathon intended? Shave your head instead!