Fake missing network connection for single app I have an app that I don't want to have network access. It does work when I put the phone in plane mode, but simply blocking it with AFWall causes it to hang, probably waiting for network replies. It seems to check, if the device has an internet connection and switch to an offline mode based on that.

Is there a way to not only block the network access, but also fake the device's connection status for specific apps? Solutions needing root or Xposed are fine with me.
You can use Xprivacy for that. In case you don't know, this is an XPosed module (available from the XPosed repo) dealing with privacy related permissions; details can be found in its XDA Thread). Instead of simply blocking access to the internet, Xprivacy fakes its inavailablity. To be on the safe side, you additionally can block internet access for that app using a firewall like AFWall+ – cannot hurt.

Side-effect: You can block/fake a lot more – e.g. your location, device IDs and other things. This can be done by defining a fixed value, or even in "random mode" – having Xprivacy either randomize data on boot, or even on each access.