I'm glad that I haven't been nominated yet. I was unaware of Fire_Thief entirely, so I'm probably not posting in the right forums to get noticed. ;-)
Anyway, even if this person only gets my vote, I'd like to nominate Triskaidekapunk. Shrike was trying to clean up the "List of free XBL content" topic and warned people to not post codes or request codes in that topic. I posted some direct links to some of the content, since I had just downloaded them and no links were provided, when Triskaidekapunk flamed/trolled me and gave away a free code. A mod deleted his post. He immediately posted the same thing again, while flaming me for reporting his post, even though I didn't. I reported this post, mostly to spite him for accusing me of reporting it the first time, and he was temporarily banned.
He then began to harass me on Xbox Live. I used to have my XBL gamer tag displayed publicly, and he used this to send me a bunch of messages, claiming that "evrybody knows ur a dick" and "u act like ur 12." He sent me a total of four long messages from three different XBL accounts before I changed my privacy settings to friends only. He even tried adding me as a friend after that to send me some more messages, but I blocked this fourth account and reported them to Microsoft.
This all happened only a few weeks ago. He's not that memorable, and I'll probably forget about him in a month or two, but he was certainly a villain, and a pain in the ass.