From a member's pop-over
Click on their username and select Block from the menu that appears. You may have to click on the More dropdown to see this option:
If the member is on your friends list you will first need to unfriend them before you see the option to block:
From your Settings
You can also block users by entering their name on your blocked list.
Click on Settings at the bottom of the left-side menu:
Select Blocking from the Settings menu:
Alternatively, you can click here
Here you can enter the username of the member you want to block and click Block User to apply the changes. Below the text box, you'll see a list of any members you are already blocking. To unblock them, simply click Unblock next to their username:
Restrictions for blocked members
Once blocked, a member cannot do any of the following:
Post notes to your account
Send you messages
Send you game challenges
Be auto-matched with you in open seeks
Join tournaments that you are running
Post in content that you created