I've observed if I say nothing (because I simply don't know how to react), opinionated people think you agree with them, which I don't. I don't care.
what I want to tell him, next time he starts ranting: 'I don't care what you think, leave me alone'.
However, I may have to work with this person in the future, so what about 'you don't have to tell me everything you think, most of the time I don't pay attention' and if he keeps pushing it 'it's tiring working with a person who has to rant to feel good, it's boring and makes me ignore you, which is a problem, because we work together.' And leave.
What about 'everybody has problems, maybe talk to a therapist? I cannot help you'.
Not if every time they start up you start talking about fossil species.
"You see what they are up to now?"
"That's a good question. Personally I am a fan of Cambrian, but who can deny how good the Permian was. Sure, the Jurassic had all the dinosaurs, but stuff Radiodonts and Hallucigenia are just so cool."
That's why I said be repetitive. It takes time. When they learn that all you do is go off on a tangent, they'll stop trying to use you for their personal sounding board.
In a way, you're responding to them in kind by reframing the convo to a subject in which they aren't interested.
You wanna be insensitive to others, and ignore the social cues that other's aren't interested just so you can selfishly rant? OK, I'll do the same, only with a subject that isn't divisive or inappropriate.
When all they ever get from you is utterly unrelated, they'll stop.
Or just change the subject completely, don't even acknowledge what they said, or make it really ridiculous. Like they talk politics, say" yea, my dog does this weird thing when politicians come on the tv" then tell a long, meaning, drawn-out, meaningless, made-up story about your non-existent dog.
Or "I don't really know anything about it, and don't really care to. Hey have you seen that email about vacation?"