They can use a different social media. That toxic social media then gets a bad reputation.

What happens when every major social network has a bad reputation?

"Start your own" isn't really a viable solution since the greatest challenge of making a social network is getting people to use it.

This is yet another antitrust issue, regulation should be put in place so that a private company cannot have own a platform with a market share large enough to become "the" public square.