Open a message that they have sent you.
On the top right of the message, click Block User.
Consider sending a message to Admin (craig) if the user has been harassing or spamming you.
A quick way is to visit the user's profile and at the bottom of the page, under the Message tab, you can send a quick message.
Or from the top of their profile, you can click Send private message from under Messages.
You can also choose Messages from the menu on the right of the page, and click Compose.