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It is determined by the algorithm a search engine uses. Typically Wiki results appear in the top tier. "If there is a specific site that bothers you so much that you don’t want to ever see it in search results ever again, there is a free Chrome extension that does just that. It’s called Personal Blocklist (not by Google). It’ll add a simple link “Block (domain)” to your search results. You can also import your whole personal blocklist."
Change search engines and see how different the results are.
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Unfortunately we don't have the ability to block particular sites in search results...and you're right, all the search engines are a bit SEO'ed with poor content that's irritating.
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