Your question is '..to disable the global menu in digikam'
What you've been doing previously is not disabling the global menu but enabling both the global & in app window menu. (which by the way should still work
To actually disable global on an individual qt4 app's you use this in various ways
QT_X11_NO_NATIVE_MENUBAR=1
For opening digikam from a menu, dash, ie. thru it's .desktop, then you simply edit the Exec= line in it's .desktop or if using unity & a quicklist the Exec= in the quicklist entry. Example on digikam's .desktop
gksudo gedit /usr/share/applications/kde4/digikam.desktop
Then use this as the Exec= line in the .desktop
Exec=env QT_X11_NO_NATIVE_MENUBAR=1 digikam -caption "%c" %i
For starting from a terminal you'd do this instead (or create an alias
export QT_X11_NO_NATIVE_MENUBAR=1; digikam
For starting from Alt+F2 - a bit more complicated, probably not of interest here
As far as why you can't use your old 'both' method, not sure. If inclined to pursue I'd start by creating a new user, logging into it & seeing if it works there. If so it's something local to your user, you can track it down if desired with a little time & effort