If you want to delete all your Yahoo contacts from your contacts app, and leave other contacts in there you can delete the yahoo account from settings within the settings for contacts. Go to Settings, then Contacts, then Accounts, then your yahoo.com, THEN Delete Account.
You can also tweak what gets displayed.
OR if you are talking deleting from within the Yahoo app itself and just individual contacts then Bam's advice is best use the Yahoo website via Safari, or IE, or Chrome, preferably on a computer (PC or Mac)