It is a waste of time to try to ban leechers, since it is impossible to distingush between normal clients and clients that have been modified to disable uploading. The only times were we can do so, is when the clients themselves readily identify themselves, however, it is (a) only some clients (b) a PITA to keep the list updated (c) easy as hell to circumvent. What we currently do is to detect aggressive clients, but to detect leechers would simply require much more work that it is worth and it would be damn easy for said mods to avoid any form of detection that doesn't have a high rate of false positives.