Suppose you think the environmental cost of flying between two cities is 100 euros more than taking a train. Then if someone prefers to fly even with a 100 euro carbon tax on flying, it is inefficient to prohibit him or her from doing so.

Only if the harm can and will be undone quickly with the collected tax.

But also, we prohibit lots acts that are harmful to others, instead of taxing them. It's also a moral thing. Values are changing and increasing share of people are seeing unnecessary emissions as a wrong.

Prohibition comes to mind. Not sure if the analogy works, but it does come to mind as something that I believe was a pretty inefficient ban where the impetus was likewise for the welfare of people.

Taxing liquor would have been a far better approach.

Flying an airplane in a prohibited way is usually very evident though. Not so much with consumer sales/trade.

Are you suggesting there could be a black market for... flying? Seems unlikely to me.

No, just that it was inefficient relative to taxing. Though tbh, I wonder why people who think taxing would be more efficient believe that. It seems like it would come with more bureaucratic inefficiencies and regulations trying to figure out how much to tax.

A clear well defined ban seems more efficient and straightforward, as long as it can be enforced. In this case - unlike for prohibition - it can be enforced like you point out.