The new GTS of X and Y allows a search feature, so if you know the name of a Pokemon (spelt correctly and shtuff) then it will know which Pokemon you're talking about, and ask for it.
People just do this to be a pain, and simply because they want an Arceus for such a measly Pokemon. (Sorry Banette. xD). You can't stop them though, so what can we do? ._.
Hope I helped. :)