Weirdest thing - I delete all the data from S Planner, and it's persistently coming back

If I go to Settings>App Manager>S Planner, stop the app and clear the data (and cookies), it's still in the app. Even if I restart the device (heck, I even took the battery out overnight) - I can still see all my appointments

Unfortunately I have about 5000 entries in my calendar so manual deletion is not an option

Any help would be appreciated