Campaigns in Draft mode can be accessed only by the campaign managers and system moderators, and therefore it is not recommended to change the campaign state back to draft after being published, and more specifically after it already contains ideas that were submitted by the community.
Nevertheless, in case a campaign was opened by mistake and needs to be deleted, moving it first back to draft is a prerequisite action.
This is done via the Campaign menu -- > Management -> Change Campaign State:
Simply click on the 'Draft' state option and confirm the transition.