I did think Kalilla's idea was interesting, but as many have pointed out, auto-moderation of that sort is often seen as heavy-handed and/or could be abused by a quorum of like-minded users. The only real way to make it viewable as 'fair' from a user-perspective (on a thread-level instead of just a post-level) might be if the threshold for reports was very high--like maybe around 20 or 30 or so unique users reporting that thread, to avoid abuse.
But having a large number of users to auto-lock a thread defeats the purpose if the offending actions were to take place during "off-hours"/non-prime-time-usage cycles when few users might be on (aside from the abusive ones.)
Further, a thread-auto-lock of this type wouldn't necessarily prevent instances of spamming (as opposed to merely posting offensive posts within a thread) as the user causing the spamming would be free to create more threads. I do recall the situation a few weeks back when a user spammed several new threads and bumped all visible threads with advertisements of his website project, which later required removal of the bump posts/new threads. Having the threads able to auto-lock for posting wouldn't fix a situation of that kind, I think.
However, it does bring up a potential need (even if infrequent) for an automated limit to thread creation. Right now it is possible for someone (willing to be banned later LOL) to create infinite numbers of threads nonstop while moderators aren't present. Maybe limiting the number of threads that could be made in a time period (3 threads in 5 minutes or 10 minutes?) might work.
I have seen very little abuse of this spammy nature over the years (except maybe on April Fool's Day LOL) although potentially it could easily happen. I can't see an easy automated system that prevents mass-*bumping* of existing threads, however, without severely limiting regular users' abilities to post as often as they wish in different threads.
Again, I don't bring those up as optimal solutions (as more human mods obviously is the best choice)--just as Kalilla mentions, if mods are not present there is little to stop people from doing the above. But while on the topic of forum improvements, maybe there is need also for "polls" and voting features. ^_-