On any Ubuntu server, wireless needs to be disabled, if of course it is enabled.
First, let's disable the wireless network from the interface file:
sudo vi /etc/network/interfaces
Add the following line:
iface wlan0 inet manual
NetworkManager don't manage interfaces configured in the interfaces file. Replace wlan0 with the interface you want to disable, if it's not the name of the built-in interface.
Then restart network manager:
sudo service network-manager restart