How to cancel Facebook ad I believe you mean to remove or delete the Campaign when you say “cancel.”

Instead, we recommend turning your campaign “off,” which is the best course of action If you don't want to see them anymore, you can delete Facebook campaigns, ad sets, and ads from your ad account.

because you may want to rerun the same ad campaign in the future and would regret deleting the ad campaign and starting over.

Open your Facebook Ads Manager and click the checkbox to the far left of your Campaign to delete it  ( the one you need to delete).