It's called the ignore function. Once you add a member to that list, their posts will not be visible on any thread you are reading.
Click on the desired username, view public profile. In the light blue bar right under their name, you will see a tab "User Lists" with a white drop down arrow. Click User list, 2 options will drop down, add to contacts and add to ignore list. Click on add to ignore list, box will pop up asking if you are sure you want to add member to ignore list, CLICK YES !!