Yes, it has been tried. But it is impractical, because of collateral damage:
- Many sites are hosted on massively shared hosts. Blocking those IPs will cause a lot of services to be unreachable, not just the one to be blocked.
- People find you with your domain name, not your IP, so a service could avoid filtering by switching subnets every day and just keep updating the DNS entry. After a certain time, the govt will be blackholing a large part of the internet.
