It's a complicated isue. I particularly dislike the idea of the downvote because of all the repercussions it may have, especially for small account trying to protest big accounts' shitty content or abuse.

The idea of total freedom of speech is as dangerous as the total access to weapons. Even if it is suported in some legislation, sooner or later it calls for some restrains. Of course, it may be easier for the blockchain to implement censorship measures than for some gun control legislation to be ever efectively passed or enforced.

In any case, downvoting, as you rightly point out is not enough when it comes to fencing off really dangerous content or expressions. And here lies the danger of freedom. We asume the majority will are good or the regulators and those in a possition of power are good and work for the wellbeing of the majority; but what happens when those in the possition of power are actually evil and could not care less about the opinions of the majority?

The blockchain is not except from this dilemma and there have to be a mechanism through which small accounts can gather to effectively protect themselves and others from possible abuses from big accounts.
Knowing how polemic the whole censorship issue is, I do not expect anything to be done about it in a long time. Self-regulation may work its miracle in this platform, but it will take creativity and organization to at least neutralize offensive/dangerous content.
I like @jadams2k18's idea (assuming it may be technically viable)