Campaign sent to over 10 recipients (under 10 recipients, we generally assume that may be a test) cannot be deleted for a period of 7 days following a campaign.
When you delete an active campaign, you also delete all stats on unsubscribes, complaints, bounces and any other reporting stats. We need to retain the campaign and stats for 7 days in order to collect reporting stats.
If you're unable to delete a campaign that means this campaign is active and will not be able to be removed for 7 days.
If your campaign falls out of the 10 recipient range and you need to hide it to organize your data, we recommend using folders to organize your account.