It actually does happen, in a social way, not at the protocol/client level. During 2013 some investment companies were buying "clean" coins only, the ones that

It actually does happen, in a social way, not at the protocol/client level. During 2013 some investment companies were buying "clean" coins only, the ones that came straight from the miners. Indeed, nothing stops from that being implemented at the node level, or by the miners. But then you need to deal with people tainting other people's accounts.