It's simply a matter of available moderator resources. We really don't have the time to look at every post and ask "is this an on-topic post?". We use a generic "ban this user and delete all their posts" feature of the board. It's really meant for handling spam, but it's also useful for dealing with the sort of morons who think is "funny" to create multiple "sockpuppets" when they've been banned from MR.
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