You cannot lock people out, but you can ignore them as MacDawg mentioned. The only downside is that if someone else replies to them, then the quote will show up.
Actually, I do not think this is true if 'quote' or 'reply' is used. The forum does a remarkable job of expunging ignored posters. The only place that I know where they still show up is in the Spy.
Oh man. This is the best thing ever. You can "ignore" an entire subform from showing up in "New Posts" this way also. Thanks for the tip! It makes the redesign that much better with these options. I wish I had learned it sooner! lol