There seems to be no way of directly (signon to icloud.com) and delete the cloud's contents directly so this is what works for me:
I save a copy of my Mac iCal's good data by exporting it or if u know where iCal's data are kept just make a backup copy of it.
Now I go into iPhone and delete my iCloud account, this will prompt msg, "This will delete your calendar data from the iPhone" I say GO AHEAD, and it's done. iPhone now clean.
I go into my Mac iCal and I manually delete everything under iCloud, this should immediately reflect into iCloud itself, so if you sign on to iCloud.com at the end of this procedure you should see zero calendar entries.
Now reload your good data from Mac archive, syncing to iCloud should commence almost immediately. Signon to iCloud.com to verify.
Now that your Mac and iCloud are on the same page, you can now go to iPhone and re-add the same iCloud account, and synching should now proceed to your iPhone with the new stuff.
