iCloud: Lock your device remotely - support.apple.com/kb/PH2700
- If your iOS device or Mac is lost, you can prevent someone else from using it by remotely locking the device with a passcode—a number that must be entered before the device can be used. To disable the lock: Settings> General> Passcode Lock. Turn it off. If your device is online, it locks immediately (if the device is a Mac, it also restarts before the lock is set), and a confirmation email is sent to your Apple ID email address.If the device is offline, the passcode lock takes effect the next time it’s online.