[Youtube] Someone posted my video that I made on their channel. How do I get it removed?

Ask nicely.

If the video is all yours (really ALL), you hold the copyright on it. (Congratulations.) Notify YouTube that it was copied and ask them to check your upload date versus the copier's to prove you were there first. Welcome to the flip side of copyright violation. Good luck.

What do you mean, posted it on their channel? Do you mean put it on their channel on a playlist? So that you can watch it when you go to their channel? That's allowed. You should be happy they did that. If they actually stole it though and uploaded it for views, then that's copyright infringement. As long as no material in it belongs to someone else, you can contact YouTube and have it removed.

You can't remove a video that isn't yours I don't know if flagging it can get it removed or not... I'm sorry to hear that... : (

The video you posted was yours? Did you have permission to use that video? If so you can ask youtube for Copyright infringement (Someone copied my creation) or for more details go to http://www.youtube.com/copyright_complaint_form?hl=en If the posted video is not yours then you can not do anything. I hope this answer will help you.