Here's a serious question; public domain is different for each country. How do you technically restrict people from non-US countries from accessing the content

Here's a serious question; public domain is different for each country. How do you technically restrict people from non-US countries from accessing the content that is legal to remix in the US? Is it acceptable to just ask them to confirm they are US citizens and then keep a cookie with that acceptance? For example, in one of the threads on Gutenberg, a German user complains they cannot access the site. Just ip blocking? Is that sufficient? Man, I love Corey Doctorow's writing. So fun.