If you think someone is purposefully derailing a thread, you should report it. If others agree with you, and also report something, the post will be hidden until the poster edits it to be back on-topic. Moderators can override the reports to avoid bandwagoning.
That said, I haven’t seen too much derailing happening, to be honest. Some threads are going long and naturally evolving into different areas of discussion, but as long as people aren’t actively derailing, I don’t think that’s a bad thing. I think we’re in a neat spot right now where the forum has only been around for one major cycle of the now-main game mode. Once the next update comes, we’ll get balance changes and new threads to replace those older ones.
Did you have any threads in mind that you think are better off being multiple threads? If you link them I can have a better idea of what you might be specifically upset about.