Now, I've looked at the script for phpBB user hide, and I am *SO* *COMPLETELY* *LOST*... I wouldn't know the first thing to start coding something of this nature.
Heck, it may even already exist...but my google-fu didn't turn anything up.
What I'd prefer it to do is to display:hide a user's icon (or in some other way not show it), and replace the body of the post with a link to the post page (possibly mimicking an lj-cut - "User Ignored: Click to view post" or something.) This is the only thing I can think of that would work with as many custom styles as LJ has.
*scratches head* Ook?
EDIT: It would work something like clayfoot's LJ collapse, but selectively, based on username. I'd personally rather not have the hover text, but that's a matter of taste.