How do I report in-game messages?

Go into the message that you wish to report. Next to the sender's name (above the individual message that you wish to report, on the right) there is a small image of a letter with an '!' overlaying it. Click on that to report the message.

You can also block the sender by clicking on the image of the letter with a 'no entry' sign overlaying it which is at the very top of the message stream, to the right of the player's name.