If you see posts on a venerable thread, and they are irrelevant to you, then just ignore and scroll on by--just like you would do on social media platforms. It's really easy to mentally filter out posts and posters, and scroll on by. Nobody really owns these boards (well, maybe Polly_mer since Polly_mer probably has access to the self-destruct button for the fora), nor does anyone own how threads are supposed to evolve. I'm actually very surprised at how someone even thinks it's even a major problem. The fluid, somewhat de-centralized, and somewhat unstructured nature of the fora is part of what makes them so useful. Yeah, someone might chime in and go on a tangent and take the thread with them for a little bit, and someone might even start yammering the same hackneyed talking points. But, so what. Ignore what you don't care to read, and scroll on.