How can I close the app?

I mean: close it completely so that it does not respawns magically after 2 minutes. The only way I found so far is to disable auto start and reboot the phone, but this is seriously unsatisfying!
And no: I don’t care about auto-uploads not working when the app is closed! Surprisingly this is one part of several reasons why I want to close it

As usual, use the task manager of your Android mobile phone and remove the app from the list.

No, this lasts for maximum 2 minutes. Then it respawns and continues whatever it was doing before

I have never seen such kind of behavior on my phone. What kind of device are you using? What Android OS is installed? Are there any background sync activities ongoing which prevent the app from being killed?

ZTE Axon 7 with Marshmallow. And yes, of course there are sync activities - one of the reasons I know that it had respawned

There were other issues like no file bigger than 1MB did download even if you told the client to sync it. An other issue was, that the uploads suddenly missed the port information and therefore were not able to get contact with the server. Flushed cache, deleted all local data, uninstalled and reinstalled the app - nothing worked. Finally I Gave up and downgraded from 3.5.0 to 3.4.2. Huge Surprise: everything is working flawlessly again and no magical respawn of the app happens after closing it.