Yeah, I don't think you have any legal grounds here, Doldol.

I believe anyone has the right to record on our servers, however we also have the right to refuse to let them play on our servers.

However since that person is using our server to promote his own channel, I believe Doldol can ask youtube to get rid of the videos.

IMO i believe it be easier if we just have a little chat with them, and explain our concerns.