To be honest, this is what happens with YouTube....a lot.
Even if you put your name on your AMV watermark (please don't do that, it looks horrible...just stick with credits) it's rather easy to crop out. Considering they cropped the top and bottom of the AMV already, I doubt they'd care much if you had your name hanging around the corner of the video.
Typically when this happens posting on the AMV and calling the thief out does the trick. They tend to be a rather spineless lot. If they block you you could have some friends post and call them out too. If you really want to, you can give them a strike.
Honestly though, most thieves don't really get too far. Even though they've stolen your video, the quality is crap, they've got text all over it, they've cut it, and it's got a whole 30 views. If for whatever reason they won't take it down, don't stress about it too much. At this level, they're practically the smelly homeless guy with the trenchcoat that is trying to show you their stuff for sale.
Also, welcome to AMV editing

You made something steal-able! Congrats! The next level is making an AMV big enough that people rip off your idea and claim it as their own.
