it were a few cases, different people where the initiators of a lengthy off-topic discussion which was getting too personal. Posts were hidden, but users felt trigger when unhiding the posts and decided to reply to them anyway.
We did discuss it with them, and usually they would agree their post was out of line, however by that time the derailment was already well underway. The latest case was 29 posts found off-topic, all replies to each other, starting with a flagged inappropriate and off-topic post. they were posted in a period of 2 days, after which we locked the thread and split out all the off-topic posts.
I think maybe a simple warning would reduce the temptation of replying, while not making it impossible. thereby reducing or slowing derailment.