I don't really believe this problem can be fixed. Yes, it's highly unfair with these players, but it's hard to find evidence as well to report these users. Perhaps J&H will come up with something for it, I don't really think much can be done against this anyways. It's still Beta as well, lets see what the future brings us, perhaps?
i remeber something use x-ray to find x-ray hackers ;) idk who it did but someone ever did that (a really smart idea) maybe for some staff?
I remember someone making a suggestion about that, it got denied. I don't expect them to add it either.
It can't be fixed. As far as I know, there are x-ray resource packs. It is impossible for staff to find what resource pack they use. You will just have to trick the x-ray users into something.
See what I do when I encounter someone with full Diamond is do 360's around him and kill him. If he's hacking then I simply press f9 to record his hax. Then I ban him with my magical skills of reporting. It sounds funny the way I said it, but it's true.
Like mentioned above, people do have x-ray texture packs...and nothing can be done about that, sadly :L