You would need to use a modifier key to cancel the buff (and decide on your own when to use it), since macros cannot change which action they perform depending on whether or not you have a specific buff (or debuff).
Code:
/cancelaura [mod:shift] Levitate
/stopmacro [mod:shift]
/cast [help, nomod] [@player] Levitate
... will cancel Levitate if you hold the shift key, cast Levitate on your target if it is friendly and you are not holding any modifier key, or cast Levitate on yourself otherwise.