A lot of the pre-mass-public social media in the 80s and 90s was actually heavily moderated and fairly civil, because the bar to entry was pretty high.

1. Afford a PC.
2. Afford a way to connect PC to an online service.
3. Depending on the service, pay additional fee for Internet specific access.

You could create anonymous names, but your ISP, your online service provider, would know your real info, so getting banned was a lot more of a threat at the time as a result.

You can raise that bar to entry monetarily, and you'd likely get a similar chilling effect. Not necessarily 'great' as a solution, but it is a solution.