Having run communities myself, I would: take it seriously—immediately—let the community know that harassment is unacceptable, support the person who reported it, investigate privately, remain neutral to all parties. Based on the result of the investigation: enforce consequences or changes—timeout, warning, ban—introduce or amend policy to help prevent it happening again, support healing of the victim and the community. These are all quite possible in private/managed communities. Not a panacea by any means, but worth doing.

As in the blog post: a campaign of targeted harassment.