Try `-site:pinterest.com` to remove any results from the website, rather than any results that contain the text "pinterest.com". A minor distinction, but one that could change your results slightly.

There have been occasions while trying to filter results from reverse image searches where excluding both Pinterest strings and domains had no effect in my experience. (There's also the problem of Google not searching for a specific string when even double quoted but that's a different one of the search engine's annoying quirks of late).