Hi! I'm not a big Mecabricks expert, but I can try to help you. If you want to report someone, PM (private message) either @Scrubs, the creator of Mecabricks, or an admin, and explain what's happened. Try not to exaggerate, and also, sometimes a user might not mean to copy, perhaps they just didn't think to ask permission.
Here is a link to a forum where someone asked about "report buttons". https://www.mecabricks.com/en/forum/topic/3101
It has the names of some admins.
Before you report a user, if this ever happens again, you can go to the model that he stole from you and ask him to please credit you as the original builder of the moc, and perhaps request that he ask you for permission before using your models again. You can also say this on someone else's models that have been stolen or used without crediting. If a user realizes that people are noticing what he's doing and aren't going to let him get away with it, he will probably think twice before stealing someone's work without crediting again.

P.S. If you have questions or requests like this again, that's what the help forums (like the link above) are for. Just scroll through for a bit, and chances are, someone's already asked that question!

P.P.S. you will probably have a better chance with an admin, 'cause tho Scrubs tries to get to things, he has a full plate.