If its on FA specifically then you can go to My FA -> Report a problem, you could then select an issue from there, from what it sounds like it would be a CoC issue if they are harassing and defaming someone, for example making a public call out post is specifically not allowed in FA's TOS
It would be better if they victim themselves fills out the ticket, as the mods prefer not to work with a third party on the issue, but if that's not possible then you could do it for them
Unfortunately, if this is being done off of FA, there's not much the admins can do, I'd try anyway but still
If it does go rather far to where the victim is fearful of their safety, then they can file for legal action for assault (threatening to harm) or defamation (wrongly hurting someone's reputation)

You must be talking about Jessi J.
Yeah, she's been spreading a WHOLE lot of false info about my good friend, and it's very quite troublesome. Seriously, though. Like a LOT of people attacked him and called him "gross" and a "homophobe" because of the awful things that she said. It was really hard to bear. He was actually very hurt by it. All he asked was that people rethink their decision for blocking him just because of his age. But, no. She went and accused him of all of these awful things, and so many people attacked and shut him out for it. And there was absolutely nothing he could do about it. Quite sad to see.
Sadly, there isn't a way to report these users. He tried, but Twitter just ignored him. Such a shame.

See, there’s a LOT of flaws in this info. I tried to keep this going without calling anyone out, but Vorchitect is really getting on so many people’s nerves. Even if she DID block him for just his age, she had every right seeing that her art is adult-oriented. It doesn’t matter if he can’t see it because of sfw mode or anything like that, he’s still a minor. Plus, he continued to block-evade and harass her behind doors. You should all just drop this, seriously, it’s just a block. Tell him to drop it and move on. Besides, her tweet never said to attack him. All that he’s getting is prompted by his own actions.

Of course she had the right to block him. He accepts that, and is perfectly fine with it. All he did was ask. He didn't antagonize her or attack her or anything. All he did was ask that she reconsider. And, like he said, you can still see all artwork by someone who blocked you. In fact, if you're watching them before they block you, you still get notified of every new upload and journal they upload.
Besides, he never told anyone to attack her, either. His friend told people to block her and make people aware of the things she said directly. She has nasty opinions, and he's not going back on what he said. There are consequences to having bad, public opinions about real people.

I don’t know what your thought process is, but it’s not right in the slightest. I don’t care what he thinks, he needs to fucking drop it. Just drop it. Not dropping it is what got him here in the first place. And if anything, HE has the nasty opinions. I’m not fighting this anymore, you’re not moving me in any way, I’ve already seen this fight from many sides and I’ve got my stance. So drop it and just leave her alone. She just wanted to be left alone from the beginning. Just tell him to stop. Please.

The way I see it, both of these "sides" did some pretty durn questionable and immature stuff. We all hurt eachother's feelings and we all went crying to our mommies.
Welp, here's the dealio: They can either BOTH delete the totally bullshit accusation tweets, and just forget this all ever happened. You and Jess and however many others will still have him blocked, and he won't try to get around any of them, and he'll be perfectly content just simply looking at all of your nice art, maybe downloading it. OR, neither of them delete the tweets, and they just sit there... collecting dust...
Hmm... I truly wonder what she'll pick... It's quite intriguing to ponder...