Such cases have to be handled one by one and more information is needed to be sure if this an off-topic post and then if it is, of course, should be deleted but...
From the moment I read this post from Theymos:
Off-topic replies have become a big problem. From now on, I will be deleting such replies without notice, forum-wide. If your replies disappear, this is probably why.
All replies must respond to the topic's original post in some way, even if they are replying to an on-topic reply. If your reply has nothing to do with the topic post, either add some content that is relevant or create a new topic.
Please report off-topic posts as soon as you see them.
I started worrying because there are for sure overwhelming many off-topic posts. Many times the main topic of the thread is forced to be changed during the discussion and I have seen such threads, participated in them and posted such answers by my own. That are very valuable side discussions, which finally in the majority of cases come back to the main topic when the side discussion finally ends. Many times OP himself triggers such a side discussion in his own thread.
I was always sure that even if the post is off-topic then, another one, for example, an answer to this off-topic post is automatically a valid post, despite it is still off-topic to the main topic of the thread.
I don't think this is even possible that every post in, let's say mega-thread could be fully on topic. Therefore, checking each case individually is crucial to be sure that the post is really off-topic and ads nothing to the actual discussion.