Over the course of time, you'll no doubt want to delete campaigns. This may be because you're de-cluttering and ridding yourself of old campaigns (some of which may have been tests), or you're clearing out campaigns which never got sent. It's good housekeeping.
To delete a campaign, select Campaigns -> Drafts from the navigation bar.
And simply click on the campaign you wish to delete and drag it to the Trash/Drag to delete tile. You'll be asked to confirm whether you definitely wish to delete the chosen campaign. Click Ok to confirm.
And that's it, the campaign is deleted. A piece of cake, wasn't it?