If i understand correctly, you want to prohibit backups of mail data from the device to iCloud. AFAIK, there's no mechnism to do this in the AS protocol (which would have to be OS- (or IOS- in this case) aware). Hence, GMS can't do anything to allow or prohibit this. There are, of course, tons of MDM solutions around which are, for sure, all able to explicitely turn of iCloud usage. That being said my impression was that an iCloud backup does NOT contain ActiveSync mail DATA anyway, but rather just the account settings. But as i've never used any sort of cloud service i'd recommend waiting for a hint from someone really knowing this stuff.