What should I do to prevent spam?
You'll probably see many messages that annoy you on any community site. People have different definitions of spam, and some people choose to message you every time they update their site while others only message to chat.
On your community site, if you see a particularly annoying neon sign of an avatar, you can also hover over their avatar and click the red x to make them disappear forever to you.
Note: This only works on your widget and on your community page.
Spam Community Messages: Messages that come to you because you joined a group are easy to stop. Simply navigate to the offending group and click the large green button that says "Leave Community".
Best Advice To Stop All Spam: Choose to only allow messages from your contacts. Many people think this makes more sense, though it does hamper your ability to make new friends a bit.