Technical point: you cannot vote to close a question with an active bounty (not even during the "grace period"). Otherwise, any combination of
- ignoring any pings from the user
- deleting your answer and comments (which prevents pings)
- voting to close
is appropriate in such a case.
The key word is help vampire. I quote one part of the article, which I think fits the site well.
Signs of Help Vampire Infestation
The chief indicator of a Help Vampire problem is the lack of helpfulness—the community may still appear to be bustling and lively, but if on closer inspection the conversation is all towards the shallow end of the pool, with moderately difficult questions going unanswered, then a Help Vampire infestation is likely.
Help Vampires are virtual bedouins. They move into a community—as soon as they sense its vibrancy and intelligence. Often they leave (“give up,” in their eyes) when they have exhausted all the resources, leaving the community itself drained and adrift.
This gypsy-like behavior incurs a secondary effect which further cripples the community, and persists even after the Help Vampire problem has passed. Often the “best and brightest,” sensing the outflux of decent conversation, retreat into Walled Garden communities which the Help Vampire can rarely penetrate. In this way the individuals are sheltered from the painful effects of Help Vampire attacks, but they also make themselves inaccessible to non-Help Vampire users as well. This effect can be the last straw that leaves the community devoid of experts and utterly without hope.