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!
Head to the landing page and look to the top right hand side of the screen for the cog symbol, hit that to be taken too the settings page.
The first option on the settings page should be a Spam filter option. Hit that and you should be all set. The spam filter will only allow messages to enter your inbox from people on your contacts list.