If you don't want to have your inteface make a DHCP request, simply edit it (right on network manager applet, choose "Edit Connections", click on the interface in question, then choose the IPv4 tab and change "Automatic (DHCP)" to "Manual" and specify your settings manually.
If you need to turn off network manager regardless, use
sudo service network-manager stop
You can specify stop, start or restart for most services.
Htorque notes that service is being deprecated in Ubuntu in favour of the
restart commands. So :
sudo stop network-manager
would also work. I still prefer using service because, as of Maverick, there's no auto-complete when you use the newer commands. A bug has been filed about this, and from that, a fix has been released, so hopefully this will be rectified in Natty this coming April.