It definitely shouldn't hide the post, unless a moderator chooses to do so for whatever reason. I've seen this done in various ways and indeed implemented it for clients' sites - my favoured approach is to use some sort of gradient in colour/saturation/lightness/opacity so that the more a thread/post gets marked as thumbs down/off topic/whatever, the less distinct it becomes on the page (whilst maintaining readability even at the the highest/lowest extremes). It can be very useful to have that sort of visual cue to indicate the quality of the post / poster and works against "troll" catchers as well as the "trolls" themselves!