There's always room for improvement in the user interface but it's actually prety hard for a user to block a domain unintentionally. It's not even an option from an individual post; you have to go to the profile. And when I click on "block domain" from a profile, here's the dialog I get -- it warns that you might not want to do this and makes it explicit that you'll lose all your followers from that domain.
[@schizanon](https://mastodon.social/@schizanon) [@mike](https://flipboard.social/@mike)