I do think there should be limits to automated decision making by these companies. It doesn't necessarily mean "no blocking" though. For example Twitter and Facebook both have published rules and an appeals process, which is two steps in the right direction.

Another good example is Google Account suspensions. These seem to be for unspecified reasons with no appeal. Given the damage losing your google account does, I think its fair to expect more from that.

Generally speaking I'd like to see tech provides be some kind of common carrier - where they have an obligation to provide equal service. However one exception that I would make is automatic discovery (ie algorithmic feeds, recommendations etc). On the balance of common good I don't think users have a right to their posts being shown to users that don't follow them or whatever. (frankly I think algorithmic feeds are the root problem of social media).