Are trolls a problem to solve at all if your goal is to promote free speech? Trolling is one of the most commonly censored forms of speech. It seems like trolls are a core feature, not a bug, in that scenario.
If you actually want to solve the problem of trolls, I think the first step would be to put away the "free speech" platitudes, sit down and figure out what kinds of speech you actually support and what kinds of speech you want to curtail and why.