internet - How to censor the internet?

How to censor the internet for example against pornography, asked ESWN and got a thoughtfull rebuttal by Imagethief. While the outside world is trying to secure the freedom of speech on the internet in China, the question whether you (meaning often the government) should stop expressions at the internet that would damage other legitimate interests.
While concerned parents might think off porn their children are confronted with, foreign and domestic companies might be less happy in the way consumers sometimes use the internet to vent their anger.
The discussion outside China mainly focuses on political issues and often narrows down to technicalities on filtering regimes - and how foreign companies assists in that limiting of free speech. I firmly believe that discussion should be broader, especially since filtering-systems reflex a very small part of the complicated reality in China.
I do not believe cracking down on the internet can be effective in the end, but a discussion on both legitimate concerns and excesses at the internet do make sense, as an excercise for educators more than engineers.