grow your wiki
A leech takes content out of the wiki and sends it to people another way, usually by email. This reduces peoples' inclination to visit it, and potentially creates confusion because information is coming from multiple sources.
Look for email with content that's been taken from wiki pages.
Talk to the leech about the importance of keeping content on the wiki. Suggest that instead of taking content off the wiki, s/he might just email people with a link to content on the wiki.
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Is the pattern truly one of removing content from the wiki? Or is it more a copy and paste behavior? I'm guessing it's the latter, because I doubt that such a person would take the time to actually edit the wiki content. If my guess is right, this description could stand clarification.