Ever been disturbed by someone in a private message who isn't in your contact list?
Well here's a simple way to stop that happening!
Open the left side menu and Tap 'Settings'.
Inside the 'Settings' menu, Tap 'General'.
Inside 'General' is the Spam Filter. This enables the user to block non-contacts from sending you a Private Message.
Tick the box to enable this, please note that for this to take effect simply log-out and log-in.