I assume you mean any reference to the device in your Network preferences? Because if it's a dongle, the easiest way to 'remove it' is to just unplug it, but I'm guessing that's not the whole story. :)
When you look in Network preferences you will see a list of devices that the OS is configured to work with, even if they are not currently available (in the case of dongles - you could simply re-insert it and you'd want the system to start using it.
If you no longer want/need the device, then just use System Preferences -> Network; select the device from the list, then click the '-' button at the bottom of the list. This will remove it from the list of known devices.