One solution to this problem that was popular in certain ancient societies was, "Round up and execute all the B's." But A) that's not very ethical, so we can't do that without becoming B's ourselves; and B) you can't tell who is a B just by looking at them, or their skin color or race or eye color or religion or whatever; and C) B's have a maximal incentive to become those who decide who is to be executed for being a B, so the whole thing is prone to MASSIVE corruption.

So the thing Liberal societies have done is create systems where we punish BEHAVIOR rather than trying to classify people. And it works, as long as we do the "impartially disincentivizing unethical behavior" thing I mentioned before.