How to Stop Syncing iPhone Calendar to iCloud
Go to iCloud under the Settings menu and turn off Calendars.
Open your iPhone's settings. Look for a gray app icon, containing gears, that is usually found on one of your home screens. If you can't find it on your home screen, it may be located in a folder labeled Utilities. Scroll down and tap iCloud. It's at the top of the fourth section of the settings menu (below "Privacy"). If you're not already signed into iCloud from your iPhone, enter your Apple ID and password. Slide the button next to “Calendars” to the “Off” position. It's the fifth selection in the iCloud menu's fourth section. All green should disappear, and the button should be white. Your iPhone will no longer sync its Calendars to iCloud. 
Open your iPhone's settings. Scroll down and tap iCloud. Slide the button next to “Calendars” to the “Off” position. 