I would not use such a tool that worked at the semantic level, but would probably think of using something that worked at the lexical level and gave me control

I would not use such a tool that worked at the semantic level, but would probably think of using something that worked at the lexical level and gave me control over the words I want to ban. For example: an extension that automatically hides HN posts whose title includes words I have manually blacklisted. Not even necessarily for being "toxic", but uninteresting to me.