You will need to remove Network Manager altogether:

sudo apt-get remove --purge network-manag*

Then do:

sudo -H gedit /etc/network/interfaces

Below the loopback stanza, add something like:

auto wlan0

iface wlan0 inet static

address 192.168.1.108

netmask 255.255.255.0

gateway 192.168.1.1

wireless-essid myrouter

wireless-key 0123456789

Substitute your details and use an IP address outside the range assigned by the DHCP server in the router. Proofread carefully, save and close gedit.

For WPA encryption, it's:

auto wlan0

iface wlan0 inet static

address 192.168.1.108

netmask 255.255.255.0

gateway 192.168.1.1

wpa-essid myrouter

wpa-psk mysecretkey

Reboot and enjoy!