uTorrent and many other torrent clients and P2P programs support IP filters in Emule's ipfilter.dat format. That way you can protect yourself from some dishonest users that try either to spy on what you share, or just over-leech data to make seeders' ratios go up fast and make them leave the swarm thinking they gave what they took. Of course there is a chance that you are blocking honest users from downloading or uploading to you but I hope this isn't that common. There are some IP filter lists on the Internet already but I am combining specific parts of other lists using BlueTack's Blocklist Manager in order to make custom one that doesn't block LAN users etc. I will update the IP filter list every now and then (I don't know if I should post here about it every time it's updated... Should I?).
The IP filter list can always be found here.
Instruction on how to use it with uTorrent are inside the zipfile.