If (as I suspect is the case) they have never had members added or been used for anything you *could* just delete them from the database. The 'mdl_cohort' table has a 'timecreated' time stamp so you just need to delete everything after the time you created them.
I would strongly advise you to take a full backup before embarking on this plan of action.
I would strongly advise you to take a full backup before embarking on this plan of action.