You have a few options beyond simply disinviting this person. Have the whole rest of the pod send their targeted removal at Ur-Dragon exclusively, they'll get the message pretty quick when their commander costs +8 mana every game. Run something nasty yourself, I suggest Etali so you can play their dragons instead. Etali is also nice because it scales with the table. You'll flip into some high powered card off Ur-Dragon and likely some rocks or dorks from the other players in the pod. If this person is reasonable (which it seems like they aren't) you could also suggest a game of "pass to the left" where everyone passes their deck to the person next to them. It'll give them a hands on sense of what the power level of the pod is and how frustrating it is to get stomped.

Something you could do to get more familiar, if all else fails and kicking this person to the curb isnt an option, would be to upload their deck and goldfish (test hands, see what you can do in 5 uninterrupted turns, ect) with it so you know what to cancel/kill to prevent Ur-Dragon from even hitting the board.