Like.... there's not any good reason to steal art and claim it to be theirs. Never.
So yeah, happened to me several times. I came after some thieves several times as well, privately at first. Then when they didn't understand or not the first time they did it and play it like "omg sorry will not do it again" and I knew about their fake apologies, I outed them publicly in a humorous way.
People started to make fun of them too, art thieves were mad at me.
I roasted them a bit more.
Art thieves and their friends were even more angry at me and spamming messages about me bullying the art thieves.
So I made more fun with even more roasting.
People brought popcorn to enjoy the show.
Art thieves were burnt to the core.
Art thieves gave up.
I was sad and told them I was waiting impatiently for their return 'cause I had way more love to give them.
Never heard about them again 
Ah, good old times when I got my art stolen. Wish it happens again 
In all seriousness, it's annoying as hell. We take time, passion to create something from our own hands, we work very hard on our arts. Sometimes there's even a deep meaning in our arts that only belongs to ourselves. Be it fanart or not.
So I have no shame kicking people in the groin about art stealing (I always stay nice but firm and only contact the thief in private first tho, it's important, some people who do that are sometimes very young kids).
It's harder tho, when it comes from business company from other countries but they often steal from various artists so I guess it's better to get in touch with those other artists and see what is doable about it.