It would be tremendously difficult, as we can generally agree whether a book is fiction or nonfiction, but 'net-unproductive websites' is unclear, and what subsidies are websites in general receiving that we could scrap? (An actual ban or tax obviously would be even more difficult to implement in a usefully Pigovian way.)

Books have copyrights, universities, direct government grants, etc.; but the Internet is famously disdainful of the former, and the mechanisms like the latter 2 are very rare indeed. (Quick: name an American poet or novelist who took a foundation or university-sponsored sabbatical to work on their website!)