And of course it would have to be possible to stop people from using their share and then selling it.
Why? We could give cap and trade another serious go, and it is in fact the closest thing available to a decentralized capitalist instrument. Even some libertarians recognize that this is the "free market" solution that they need, even if it's not the one they think they deserve right now.

Of course, then we get firms speculating off of derivatives of carbon shares, but that's a separate issue.