I take it you aren't using a network manager program?
If you look in the file /etc/network/interfaces
you'll find the configuration for the network connection. Edit/comment out the ethernet connection.
Or if you are using a network manager, you need to edit that file so it only reads
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback