Telling the truth about liars
The problem with Cal Thomas' argument here is he equates calling someone a liar with vulgarity. But some people actually are liars. It is not uncivil to call someone a liar, if he actually is a liar - especially when a liar is trying to destroy someone personally or professionally with his lies.
The breach of civility is protecting and enabling the liar by scolding those who are telling the truth.