I won't name this guild's name, or tell you where they're from, since I don't need them tracking me down in game or getting a bigger head. What they do to annoy the frack out of everyone (and that was my obligatory Battlestar Galactica reference for the week) is pretty lame - they post updates to bosses they haven't killed. For instance, right now they're claiming they've downed Brutallus. They even have a clearly photoshopd kill picture to prove it. However they haven't, and if you look at their armory profiles no one in the guild is above Kara level gear.
This guild will go and post in progression threads, taking up page after page of space. Eventually these threads have to be closed since they become full, and new ones started. However the trolls are right there to start everything again. 99% of the people on the realm ignore them, however there is always the 1% that gives them the attention they desire.
What can be done about this?
We've tried a few things. First, we tried just including them in the progression threads. "Yeah it's funny guys, we see you, now go away." After that just resulted in tons of flames and general annoyance, there was an attempt to switch to using WoWWiki.
WoWWiki is great in that it allowed us to update and correct the trolls as they edited in false information. And when the trolls went overboard and started doing too many edits, they got a temporary ban. However, that didn't do much to sway them and they started defacing the realm's Wiki page again after a few days.
I think the only way to really get around these people is to have better realm forum enforcement by Blizzard. We rarely see them step into a realm forum, or if we do, there is never any noticeable presence like there is on the Customer Service forums or the Wrath of the Lich King forums. If they were able to step up their enforcement, the effect trolls like this have on a community would be decreased. It would not eliminate them, but at least long standing threads wouldn't have to be trashed because some immature folks think they're being funny.