This is directly from Carnival's web site on their FAQ page:
"Can I book a non-smoking stateroom?
Staterooms onboard are not designated as smoking or non-smoking. Guests have the choice if they would like to smoke in the cabins or not (smoking is not allowed in Spa staterooms). All cabins are fully refreshed after each cruise to guarantee incoming guests’ comfort and enjoyment. If upon boarding you are not completely satisfied with the freshness of your stateroom, simply call housekeeping and they will refresh your room again for you."
Link
This specific question is near the bottom of the page (about a dozen up) on the "Life Onboard" tab.