To people calling for anything like Wikileaks to be shut down
I just go "gah". If people are totally incapable of evaluating the quality of their sources then what the hell are they are they doing, well, passing any kind of judgement or commenting on anything really.... Then again I might have too much faith in the capabilities of the average journalist and population?
Anyway you won't prevent people from slandering other, calling them names, spreading rumours and so forth. The only thing which the internet does is to potentially expose all this private gossip which has been going on since ages immemorial to public eyes.
You could lock down the entire affair, vet every line of writing on it and so forth but it would kill so many advantages of the net that it just wouldn't be worth doing that (not to mention the general outcry by most users). Unfortunately that would be the only way to prevent malicious rumours from ever surfacing, and as stated above once they're out there they tend to stay out there.
One thing that might be useful would be to introduce a "judgement of Fact" or whatever you might care to call it. Basically, If you identify a slanderous comment which you want to address you can bring it to the attention of a judge or arbitration panel and contest it. They'll evaluate the evidence for and against and issue a ruling. It would e a way to set the record straight and if, say, a potential employer/client were to ask about something they read on the net you could point to that judgement. Details would need to be nailed out etc (who would pay, who will argue for the potential slanderer, etc).