Yes he can but he can also start a new world that has perfectly flat terrain.

But there are 2 problems: The clouds are not very convincing and you have to be a registered licencee to use these pictures.

and to turn off clouds you turn atmosphere off or you can just chance cloud settings

To Calined:
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Yes, I need a little bit of time to click that..and click that
to find the correct settings......OK, I can render my clear blue
sky now.

EDIT
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To Bilbo_Bogimps :

What I'm doing is click "Cloudscape"->Turn Off "Density contrast" and
"Density shift" then "Edit cloud color"->and turn R/G/B to 0, finally
uncheck Land.