A campaign can be deleted at any time from the campaigns screen. Select the campaigns you wish to delete on the left hand side.
Once selected this will change the top menu. This will now allow you to either Copy, Archive, or Delete your selected campaigns.
If you have scheduled a campaign, that has not yet been processed, you may still be able to cancel this. In the same 'Campaigns' list, a red 'Cancel' button will appear if this is possible. Once cancelled, you can then select and delete the campaign as above.
Once the campaign cancellation deadline has passed, your campaign cannot be cancelled OR deleted.