Completely remove an application and its data on iOS 8
No, there is no other ways of erasing app data. As a developer you can't save data out of your bundle, and when you delete an app on your device, all bundle content is deleted.

Regarding the logged issue, one posible answer may be that the app is identifying your device and saving your configuration on internet. To ensure that is your case, you can delete and reinstall the app, turn off wifi and celular data just after install and without open the app. Then open up and check if your info is there.