Should address OffTopic material (either embrace it or eliminate it--at a minimum, set clear guidelines as to what is OK and what is not).
Note that spam and vandalism have very different motivations, although they are similar in outcome. A well-crafted solution is needed for each problem, but discussion is likely to be more productive if we don't confuse the issues. WikiSpamSolutions should be addressed on that page, not here.
Most proposals either attempt to make trolls want to leave on their own, or to forcefully remove them.
Ideas to make trolls want to leave on their own: