Yes, there seems to be no way to make it clear to Google that we want to never see certain websites in our search results. Yet, Google claims they need our information to "improve our experience".
If Google wants my information to improve my experience, I'd love to be able to vote search results up or down. Or entire sites, like pinterest and content farms.
Wasn't that a thing? I remember a +1 button somewhere on Google Search
There was a thing called SearchWiki where you could adjust your own results. It didn't last long.
-pintrest should be a search extension.
For what it's worth, DDG image results doesn't get spammed by Pinterest. While my browsing is a drop in the ocean compared to Googles market share, using a Google competitor is as clear a signal as one can send that you're unhappy with the Google service.