However, the best solution I've seen is a voting system (like Reddit), where trolls get down-voted and effectively ignored.
I think free speech goes hand-in-hand with the Bitcoin philosophy and places like Reddit turn into circle-jerks where only the popular opinions ever get seen and anyone expressing a contrary opinion is effectively silenced via downvotes.
Apparently freedom is a great concept until it becomes a minor inconvenience, right?
There is plenty of free market demand for well moderated forums. Freedom of speech is not an obligation for others to listen.