In Boot2Qt the demo application is launched on boot via systemd service. In this case the service should be called qtlauncher.service. You can disable this service from running with the following command:
systemctl disable qtlauncher
Upon rebooting your device the demo Qt application should no longer be running.
As for auto-running your application you can make use of the same systemd service method to enable your custom app to run on boot. It is suggested to use the contents of the qtlauncher.service file as a starting template for your custom app’s service file.
Also as a side note if for some reason the Qt Demo service is not called qtlauncher on your system then you can list all the enabled systemd services with:
systemctl list-unit-files | grep enabled
Then you can locate and disable the right service from the above list.