When I create a thread and have to reply to 3-4 people at once who replied on my thread, I usually create two continuous posts. Things look smoother that way.
As far as I know this is against the rules. You should reply all of them in one post. In my opinion, the best thing you can do is to separate them using by a line.
Well, whenever you feel like someone is spamming by making continuous posts, just report them. That's the best solution. No matter posts are old or new. I have seen people getting banned for posts made in 2015-16.
I don't think a user is banned due to multiple posts made in 2015-16. At least I am sure they don't get permanent ban. Those users have likely been banned due to plagiarism.