The C2 arcade is designed to allow anyone to embed it freely on their websites. At the moment, the only way to prevent this is to host it on your own website, not on the C2 arcade.
Personally , I don't mind it (my game's up on like ten of those), I don't see any harm, but if you are interested in making money this could be a problem.
(Also, the only ads I see are Scirra ads, which come when someone embeds from the arcade).