On thread locking in general... I think that threads should have a warning before they are locked, something like 'If you carry on this way, this thread is going to be locked'... that gives people who are actually interested in the topic a chance to get back to the topic and might make them see where their posting has been inappropriate. Also, trolls/stirrers might then back off when they see that mods are taking the thread seriously.
There might be circumstances in which mods feel threads need to be locked straightaway without a warning, in which case there should be an explanation, e.g., 'All these racist insults are unacceptable on the forum, thread locked' (just as example).
Locking threads with no warning or explanation, while it probably seems crystal clear and perfectly logical to the mods, can cause resentment because some people might be unsure of why it was locked (if there is still the possibility to get some constructive on-topic discussion out of it) or they might feel they didn't get the chance to have their say. I realise that the mods can't pander to everyone but should lock inappropriate discussions, but on the other hand the forum membership does need to feel it is respected, otherwise we'll end up not
having a membership.