It would be nigh on impossible to spread a virus using a torrent, due to the hash key that stops anything that doesn't match its hash being used. People can't just magic anything they want onto your system using Bit Torrent.
You ban an IP for sending stuff that fails the hash check because its useless data, if you kept downloading useless data from them it slows the swarm down, it makes your download longer, so its useless. Thats why you ban the IP. Shadows Client, along with uTorrent(
AFAIK) both ban clients, I think ABC does as well.
What I dislike from you is the spreading of mindless fiction, when you make it fairly clear you know little about the of bit torrent or P2P as a whole.