Have anyone noticed that the reason why some persons post go off the topic is because they don't read the entire op writing before they reply to a thread, some persons respond to a thread based on the caution of the topic and that is why we meet up different opinions or suggestions that is not in lines what the OP points of view.
Secondly, when someone have written what you want to write in same thread you have to give off, instead of making a pointless point, I know very well that many users in forum make off topic because what I outlined.
Actually there is nothing wrong if after stating your point on a particular thread and there are still other reply or quotes that you feel needed to be corrected, perhaps you have every right to still write on that particular thread so it shouldn't be regarded as a pointless discussion or off topic, however in most cases of speaking off topic could be caused by the Op because most Op's title is normally different from there statement or the points they are trying to make and one thing about most people is that they find it very difficult reading through everything the OP has to say before making there post, so perhaps instead they prefer posting based on what the title is, that's why most thread has different dimensions of discussion, So however in regards to pointless discussion as you presume I think the best way is to align the title and message you are trying to pass together so that people will not get confused.