The peer to peer (P2P) file sharing applications have allocated a significant amount of today's Internet traffic. Among various P2P file sharing protocols, BitTorrent is the most common and popular one that attracts monthly a quarter of a billion users from all over the world. Similar to other P2P file sharing protocols, BitTorrent is mostly used for illegal sharing of copyright protected files such as movies, music and TV series. To impede this huge amount of illegal file distributions, anti-P2P companies have arisen to stand against these applications (specially the BitTorrent). To this end, they have begun to fire large-scale Internet attacks against BitTorrent networks. In this paper, we are going to actively measure the impact of the piece-attack against BitTorrent networks. Our measurement is divided into five scenarios in order to figure out the constraint factors that influence the success of the attack. To be able to evaluate the attack in different experiments, we defined attack effectiveness to quantitatively verify the success of the attack. Based on the measurement results, we discovered how it is possible to achieve significant outcome with modest amount of resources used by the attackers in hampering the illegal distribution of files in BitTorrent networks.