If you are willing to invest some effort you can block somebody using your firewall (just block his IP). I do this once in a while when I can identify a client that uploads corrupt data.
Speaking of which... maybe we should think about implementing better measures against corrupted transfers. Iirc, mldonkey has a pretty smart way of dealing with this. If a chunk turns out to be corrupted all clients who uploaded parts of this chunk are flagged and not mixed with 'clean' clients for other chunks until they have proven their 'innocence'. Once it is confirmed that a client reliably uploads corrupt data it is banned.
This is just what I remember from the top of my hat and I think the actual system was much more sophisticated. Anyway, back in those days I never had any issues with corrupted chunks, and that was long before AICH...
