Is there a way to force apps to use internal memory only?
you say your phone is rooted, it is then possible to change the default installation location that the packagagemanager uses to install apps. this will only work with apps that allow it, as others have already pointed out, apps may choose to ignore the packagemanagers recommendation and install to internal/external anyway even if you choose the opposite. also, big game files in the obb directory are stored where the obb directory actually is located; this is dependent on which phone you use.

to change the default installation directory:

1) use terminal emulator from google play (or, alternatively, enable usb debugging and use adb)

2) in terminal emulator, enter:

su


accept the superuser request, then try either:

pm setInstallLocation X


or:

pm set-install-location X


where X can either be

0 = let packagamanager decide

1 = install on internal by default

2 = install on external by default