You'll be automatically logged in to the app every time once your account is created. If you need to, you can log out of the app by navigating to the Account tab. Scroll down to the bottom and tap "Log out of ev.energy".
This will simply log you out of the app - it will not effect your schedule, and we'll continue to smart charge your car in the background based on the last ready by time you set.
If you want to break the connection between ev.energy and your car or charge, you will need to delete your account.