Then someone else produces and sells it. Exxon doesn’t control market demand. If companies could control demand for their products then they would make infinite money by just controlling the demand to be as high as possible.
Either way Exxon totally has a way to reduce the emissions of their customers: by reducing their customers. They may go to other producers but this is hardly the only thing activists are trying. It's not an impossible request and could be a useful component of a larger strategy.