It's actually pretty easy. Just cite Wikipedia (after doing your due diligence to insure that the page you're citing is unbiased and well-sourced, of course). If they respond by telling you that Wikipedia can't be trusted, you can ignore anything else they have to say and go on about your day.
You're probably thinking, "Wait a minute, Wikipedia can't always be trusted. They've actually got a point." True. Kinda. You're right, Wikipedia is a terrible source for recent information, and the very nature of the beast makes it possible for editors with an agenda to introduce bias into controversial pages. They key word in the fictional statement I'm pretending you made is "always." There's a huge range of subject matter for which Wikipedia is completely trustworthy.
When someone brings up the "Wikipedia can't be trusted" meme (which almost always happens when a Wikipedia page contradicts their own argument), what they're really saying is "Wikipedia isn't always 100% accurate, so I have no way of knowing whether or not to believe this page." Since a good Wikipedia page includes source citations (often with links that you can follow and use to evaluate the source right from your computer), someone who says this is basically admitting that either they can't be bothered or don't possess the intellectual tools to critically evaluate a resource for themselves. If they can't figure out on their own whether they can trust something as clearly cited as a typical Wikipedia page, they're not smart enough to waste your time arguing with.
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