Likening your own concern for quality with the reported megalomania of some of Wikipedia's inner clique seems a bit far-fetched. For starters, it's hard to imagine that there's much to be gained from skewing a particularly topical page on your wiki! However, the coverage of Wikipedia does serve up some interesting questions about how information should be managed in such a collaborative medium. Who is right? Who decides they are right? How do you prevent a vocal minority skewing the process?

In a word: you! Collaborative techniques are not plug-and-play solutions - they take time (and often considerable effort) to adopt. Until the team is self-managing you'll have to manage them - and that might have to be quite strictly to avoid bad habits forming. It's not democratic, it's possibly not even meritocratic, but it'll get there eventually.

That said, I'm a fan of freedom of expression, whether it's in wiki entries or code commentary. While you might argue that an off-topic entry is a potential maintenance risk it would have to be weighed against the potential for stifling enthusiasm. I'd be more worried about the use of a survey plugin on a wiki of this nature. At best it shows a naive view of statistics and response rates! At worst it may indicate someone who can't interact with the team more directly. It's probably just a bit of tyre-kicking, though.

In my experience these things tend to iron themselves out over time. Whenever an opportunity presents itself you instruct someone to "put it on the wiki". Concise, relevant content will increase. Irrelevant content will be sidelined - whether you choose to remove it is perhaps academic at that point.