sudo apt-get install bbswitch-dkms

then

gksudo gedit /etc/modprobe.d/blacklist.conf

and add the following lines

# Blacklist the alternative nvidia module 
blacklist nouveau
# Blacklist the original nvidia module 
blacklist nvidia

then

$ gksudo gedit /etc/modules

and add the following lines

# Switch off discrete GPU 
bbswitch load_state=0

at last update the initial ramdisk by running

$ sudo update-initramfs -u