What's the threshold they have to meet to ban? Half of them, give or take, will probably not be able to recognize the lie, and a sizeable portion of them would likely not be convinced in deliberation. It's also subject to nullification, e.g. "I know it's a lie, but it 'owns' the people I don't like"

Yep, it's hard. Let's deter it in the first place with imprisonment for the CEO if convicted. Ads will be amazingly informative.