When I enable the "screen keyboard" from the accessibility next to that the GNOME onscreen keyboard is started onboard is also opened. Is there a way to prevent this?!

When I enable the "screen keyboard" from the accessibility next to that the GNOME onscreen keyboard is started onboard is also opened. Is there a way to prevent this?!

This a known bug. But if you're planning to use only Gnome Shell instead of Unity you could uninstall onboard
sudo apt-get remove onboard
If you do this you won't be able to use onboard on LightDM.
Anyway, it seems possible to use LightDM with Caribou, but the unity-greeter is coded to use Onboard, so it would be necessary to write a patch for unity-greeter.
EDIT. Another workaround is to remove onboard autostart from /etc/xdg/autostart without removing onboard.