Moderation is somewhat subjective, in many cases, though if topics start drifting into prohibited topics (or start there ) it might be easier not to individually moderate posts. Then, you add that there are multiple moderators on at different times, tracking can become an issue, and again, the simpler solution may be to lock a topic.
As far as I am aware, the only way for a poster to be blocked from a specific topic is for the OP to block them completely. Also, the OP can request a topic to be closed but I wouldn't think any other poster could request it successfully unless the topic was veering out of control anyway.
Why do moderators close entire threads because of one or two posts within that thread which they deem to be inappropriate? Surely the course of wisdom is simply to remove the offending posts and issue a warning. If the warning is ignored then ban the person from that specific thread. It seems not only unjust but entirely disproportionate to punish everyone who contributed to a thread because of the actions of one or two individuals.
This has happened here on numerous occasions, a thread was doing awesome and some tin hat forum dictator chances along, decides that they do not like the tone of the thread and locks it entirely so that no one can contribute. Does that seem rational to you? does it seem fair to you? what about reasonable? How can such an action be justified? I have even seen instances where some crybaby unable to substantiate their claims have asked the forum moderation to close a thread. Again does that not seem rather biased? unreasonable? even insane?