Is it possible to accurately define this in law and policy, such that actions against it can be fairly enforced?
If you spread lies about a group that demonizes them you put members of that group at risk.
For instance, if I started a viral rumor that white male computer programmers all keep sex slaves under their houses and enough people believed it a lot of the people on this website would suddenly be living much more difficult and dangerous lives as lots of strangers would assume that they were evil monsters who needed to be put down for the sake of children without even getting to know them. The murder rate against white male computer programmers might even rise noticeably as they became a target for random violence by people who wanted to do a "good deed" on their way out of this life.
That seems to me a highly disproportionate infringement of this person's freedom of expression, as well as being completely pointless. If you're going to have the police arrest people for posting conspiracist idiocy on the internet, they'd have no time left to investigate real crimes that meaningfully affect people.
If it reaches a point of causing public disorder or harming the general welfare of a democratic society, then why not arrest?