Exactly the same issue confronts ISPs required to filtering content under the UK Government's 'pornwall' scheme to require broadband subscribers to opt in to receive certain kinds of content.
The idea has been heavily criticised on a number of counts, including inadvertent censorship of legitimate websites, but others question whether it will even work.
Using proxies is getting easier and easier; last month a Chrome filter called 'Go Away Cameron' (GAC) designed to bypass the UK ISP restrictions received plenty of free publicity. There are many other tools like it.
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