I have wondered why is it that profilically obsessive vandals, like M and L, can return again, again, and again...

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They return because they have become emotionally invested in Wikipedia. This is an inverse of the attachment that happens to our regulars; it is destructive rather than constructive. Wikipedia has a history of making persistant troublemakers into "super-villians" with infamous reputations.

For those vandals, are they still given a certain amount of time in hope of their possible reforms?

No, those people have used up any grace they've been extended. The problem is that IP bans do not work on anyone with a strong attachment to Wikipedia and a tiny amount of technical knowledge.

Stephen G. ------- Wikipedia: The Free Encyclopedia http://www.wikipedia.org