Nowadays there are some users or companies or maybe goverments have clients to monitor torrents. ther are like:
can we somehow block the only "client_test" client by default?

I'd also like to be able to block some clients. It would be great if I could regex match peer id and/or the client name from extended handshake. There are more and more clients that download sequentially by default, I don't like to connect to them.

Why not simply start a list of your item to block, grab their ip addresses and then use ipfilter and block the entire ip domain?  Just a thought, probably would be a bunch of work but seems like most of the companies you want to block already have a blocklist, ipfilter, whatever already created

Countries (associated with governments) can easily be blocked in the ipfilter...for example I block china.

Anyway, just a thought...