unsyncing an iphone 4
What data is syncing with the iPad?  It sounds like the other person logged into their iCloud account on your phone which would give you any or all of: mail, contacts, calendars, reminders, safari bookmarks, notes, passbook, keychain, photos.  Is that what you're seeing?  If so you need to log your phone out of their iCloud account.  Go into Settings -> General -> iCloud and at the bottom you'll see "Delete Account".  

If the other person enabled Find My iPhone with their iCloud account you MUST have them disable it on your phone, which they can do that directly on the phone or on iCloud.com.  There's no other way to disable it.  If you try a full reset the phone will boot back up with the Activation lock (aka: iCloud lock) switched on and it'll be nothing but a paperweight until the other person unlocks it.