How to remove "Reminders.app" from Mac OS?
All the default apps are protected by System Integrity Protection.


  To disable System Integrity Protection, you can do the following:
  
  
  Boot into the Recovery HD by restarting whilst holding ⌘R.
  Open Terminal (from the Utilities menu).
  Run the following command in Terminal:

csrutil disable

  Restart.
  


Source: Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations

You can re-enable it once you've deleted the app.