Trolls show up to manipulate the price down so they can buy at little lower price, whether because they failed to buy enough when the price was lower or before a planned pump. 

http://www.reddit.com/r/blackcoin/comments/35pjyy/care_feeding_of_trolls/

I would have to think that once they're well-known to be trolls, their posts have very little or no negative effect on a coin's price.