Previously the WTO ruled that USA had imposed illegal trade barriers against Antigua that violate the GATS treaty by attempting to criminalise any website in any country that takes wagers from Americans[0]. I'm pretty sure any site blocking effort would violate the same treaty, but those cases can't be taken to the WTO due to the USA blocking appointments to the WTO appellate body since 2019 [1]

[0]https://en.wikipedia.org/wiki/Antigua#Online_gambling

[1]https://en.wikipedia.org/wiki/Appellate_Body

The US lost in the gambling case because their restrictions on foreign websites were stricter than those on domestic ones. The GATS doesn't prohibit countries from regulating trade, they only have to do so in a non-discriminatory manner. Spain isn't blocking foreign websites for copyright infringement that would be legal domestically, so they're in compliance with their obligations.