You can add forum members to both your Buddy List and Ignore List here. Adding members to your buddy list will allow you to easily send PM's to them through the PM Compose page.
Adding a member to the Ignore list will block any PM sent by that member to you, and will replace every post from that member for the following message:
The contents of this message are hidden because <blocked used> is on your ignore list.
You can still read the post of that member by clicking the Show this Post link on the right side of the window.
You can remove a member from any of the list at anytime just by clicking the little Delete icon next to the member name.
You can also see the online/offline status of a member on the list here.