Unhelpful sites are banned by users marking those pages.

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Those social media sites are still banned in North Korea, says the official.

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Meanwhile a 17-year-old from Suffolk has been banned from using social networking sites for 12 months and ordered to observe a three month overnight curfew for using Facebook to encourage people to riot during last week's disorder.

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Maureen O'Gara remains banned from publishing articles on Sys-Con's 16 sites.

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Many companies use filtering software, produced by companies such as SurfControl and WebSense, which uses a watchlist of sites and stops employees reaching any website on the banned list.

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