Also known as the Human Emergency Brake. Someone who takes the wind out of the proverbial sails. Found on any sort of conversation thread (Facebook, message boards, etc), they are the one who always has the last comment. This is because they abruptly bring the conversation to a screeching halt with boring, lame, unfunny, or inappropriate comments. Or because they are just generally disliked by others and nobody wants to engage them in further discussions.
We were having a funny conversation on my Facebook page and then Jonny, the Human E Brake, chimed in and the whole thread just died a miserable death.