Here you go. I tried to keep the tone of your OP. (You can use this for future reference.)
“I am very bothered by the fact that, when someone starts a topic, other people start ranting, whether on topic or (mostly) not. I am not interested in reading all that; therefore, I am posting this for those who want to rant. Just state your topic, and then the rest will be more enjoyable to read. Thank you.”