I'm not sure I agree with your premise. The right to free speech only guarantees that you can't be punished by the state. It says nothing about the reactions of other people. In this world, the moderator would not be allowed to ban you. However, the right to free speech doesn't mean you are free from hostile responses or an overwhelming counter-opinion. Your rights aren't being infringed upon just because everyone else disagrees with you and uses their downvoats to make your opinion hard to find. Libertarianism supports the free market of ideas, and so if you can't stop people from mass downvoating your through persuasion, then you have failed in the marketplace. It more than likely means your ideas are shit. In the case of Reddit, they have essentially government enforcement to make sure that the truth is suppressed whereas on Voat or the Chans you find that the dominant opinion trends towards to actual truth.  If, in your example, the harassment exceeds simple disagreement, then they may have violated the Non-Aggression Principle in some other way in which case they are guilty of a different law than violating your free speech. They are possible guilty of physically harming you or non-consentually coercing you in another way. The right to free speech doesn't guarantee your right to be heard or to be considered correct because both of those things violate the rights of others to choose not to hear you or to consider you wrong.