The Police Intellectual Property Crime Unit (PIPCU) at the City of London Police has begun replacing advertising on copyright infringing websites with official force banners, warning the user that the site is currently under criminal investigation.
More info can be found here: http://www.cityoflondon.police.uk/advice-and-support/fraud-and-economic-crime/pipcu/pipcu-news/Pages/Advert-replacement.aspx