For creating a sky block map or something like that, it is helpful to create a complete empty world (and than later to add a block with a program like MCedit).

The easiest way to do that would be to create a new flatland map and to remove all layers under "Customise". After do that I clicket on "Crate" and I spawned in a normal flatland map.
I know that I can do that under "Presets", but I think it's a bug, that it doesn't work by using the first way (see above).