Remote lock
You can lock your device from another device using a text message. To enable your device to be locked remotely, and to define the lock message to be used, select 
> Settings > Settings > General > Security and Phone and SIM card > Remote phone locking . When you have entered the message, select OK. The message can be up to 20 characters long.
To lock your device, send the lock message as a text message to your mobile phone number. To unlock your device later, select Unlock, and enter the lock code.