Well guys, welcome to YouTube fame. Getting your videos copied off of YouTube is part of the YouTube fame fortune- Don't like it? Then take your talent elseware. You videos being stolen presents a good thing....
Why do I say that? Because it means they're wannabes, they want to have the same talent or same content as you. They're jelly of you! (jelly=jealous)That's my take on it if someone were to steal my vids.
If they pass it off as their own, then that's where it stops and you simply just DCMA that sucker. YouTube can do remove it if you do a DCMA report correctly.
Of course watermarking works too.