Euphemisms for pornography, violence, and hate

I vote for specificity so that users know exactly what is prohibited and you aren't passing value judgments on anything -- unless the site's mission is passing value judgments.

On second thought, "hateful" denotes a value judgment, but I guess it's hard to avoid that if you want to be clear about what is and isn't allowed.

Indecent or objectionable is a possibility, but it leaves the door open to whatever interpretation a user decides to make.

“Explicit, inappropriate, and offensive content” probably covers all your meanings but is kind of vague.

I assume this isn't a legal declaration? If you can use an informal, assertive tone, I would consider:

No mean-spirited content, please. It's bad karma. Be nice.

or possibly:

No nasty content, please....