When someone new is banned, either because they are spamming or a new account of someone who has already been banned, sometimes mods will delete all their thread and posts. I remember at least one case of a thread being deleted because it's op was a sockpuppet of a banned member. This isn't always the case though, and with people who have a lot of posts it doesn't really happen, but then again, I know of one case where all the posts of a relatively long standing wp member were deleted.
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To build may have to be the slow and laborious task of years. To destroy can be the thoughtless act of a single day. - Winston Churchill