Heya! I actually made a topic on plagiarism last week haha. I have unfortunately experienced this with my ocs and concepts where people in the past have taken my ideas and repainted them with their own ocs. It understandably ignites negative feelings and I'm so sorry someone plagiarized your fan fic. People need to respect fan fiction more. ;w; What I have personally done when I've caught people plagiarizing my writing is I actually take screenshots of when my work was uploaded and then theirs. (Glad I have my novel site that's existed for years! It's where my novel was originally posted for about 5 years.) ...That way there is proof. I don't think I'm Stephen King but I am aware my writing style is unique to me. I'm not dumb, ya know? I can tell when someone is using me as a template. I'm sure that's how you feel about your work too. And I also agree with the others, you definitely also should try to talk to the person.
I think an issue is sometimes people don't understand what plagiarism is; if you're taking someone else's paragraphs and slightly tweaking them... You're stealing. That's plagiarism. If you're taking someone else's concept for a character? Or a concept for a story? And just slightly tweaking it with your own ocs? You're stealing!
I'm a graduate student so I take plagiarism super seriously. Plagiarizing a research paper will get you expelled in my university. (And it's rude to steal people's hard work on their writing.)