Why do you need to disable EFI variables? There may be another way to accomplish your ultimate goal without messing with the EFI variables.
noefi kernel option should indeed disable the EFI variables. If you've only tried it once, you might try it again; perhaps you mistyped it. I just tried on an Ubuntu 12.04 installation in VirtualBox, and
noefi disabled the EFI variables support for me. (
efibootmgr no longer worked and
/sys/firmware/efi was missing.) You might also try placing it somewhere else amidst the kernel's options.
Another option is to boot using a BIOS-mode boot loader rather than an EFI-mode boot loader. If the USB device you're using is an Ubuntu installer, I believe it includes both types of boot loaders, so you should be able to use your firmware's boot option or possibly rEFInd 0.4.6 or later to select your boot mode. If you're using a custom USB flash drive, you'll need to install a BIOS version of GRUB manually.
A final option might be to remove the
efivars kernel module, as in
rmmod efivars, after booting. I tried to test this, but my test system uses a custom kernel with that module compiled in directly rather than as a module, so I can't guarantee that you'll be able to remove that module.