You will never be able to block them from viewing stuff they want to see. They’ll either do it through their friends devices, on other WiFi connections, or at school where networks are hilariously open or easy to bypass.
The best thing, and frankly the only thing you can do as a parent is to be engaged with them. Make them think critically on subjects, and if something they parrot back is nonsense, call them out on it. Cast that seed of doubt in their mind. If they choose to continue to watch stupid shit, that’s their choice, and it’s only worth stepping in if it’s actively dangerous.