Peer-to-peer (P2P) applications, most notably BitTorrent (BT), are generating unprecedented traffic pressure to the Internet Service Providers (ISPs). To mitigate the costly inter-ISP traffic, P2P locality, which explores and promotes local peer connections, has been widely suggested. Unfortunately, existing proposals generally require that an ISP control the neighbor selection of most peers, which is often not practical in a real-world deployment given there are noncooperative trackers. In this paper, for the first time, we examine the characteristics and the impacts of these noncooperative trackers through real-world measurements. We find that tracker blocking has the potential to address this noncooperation problem, and help the ISPs to control more peers for traffic locality. Yet, how to guarantee torrents' availability at the same time remains a significant challenge for the ISPs. To this end, we model the tracker blocking problem coherently with torrent's availability, and address it through a novel selective tracker blocking algorithm, which iteratively improves traffic locality with a given availability threshold. Our trace-driven evaluation shows that our solution successfully reduces the cross-ISP traffic in the presence of noncooperative trackers and yet with minimal impact to torrents' availability.