Arbitrary is fine - there’s a reason we have humans do this. But any enforcement of bad rules will always suck.
“Be nice” is a bad rule.
“Be nice” is a recipe for failure, and it always winds up protecting mildly cautious assholes. If you see someone reply ‘so you think [insane garbage unrelated to parent comment]?’ and the accused shoot back ‘shut up,’ and you only remove the person brushing off that troll, your forum is for trolls. That is who you’ve protected. That is what you’ve encouraged. That is how things will go.
If you think the right answer is to always expend great effort peeling apart that disinformation, you do not know what trolling is.
It’s outright insane, in communities about serious topics. If your forum’s about knitting - yeah, you can expect and demand televisable language. The vibe is properly casual. But if you deal with politics then you’re going to get people being called subhuman, and if you don’t come down ten times harder on sneering bigots than their pissed-off victims, you’re not preventing abuse, you’re enabling abuse.
Trying to enumerate all the ways someone could deserve a time-out is a fool’s errand. You can be mercilessly rude with nothing but a thumbs-up emoji. Or “Great Save!” More importantly - some vitriol is justified. Be human, god dammit, and spend thirty seconds figuring out if someone’s being a crank or merely dealing with cranks. If you think there’s never any reason for one user to tell another where to shove it, then you are wrong and you should quit.