How do I disable sonos WiFi/sonosNET when running ethernet?

Sonos does not support turning off WiFi. Google has the procedure, but we cannot list it here.

If you have a speaker that is wired, you're correct that the wireless radio is in fact still powered on. Sonos uses a local network switching protocol to determine the best route for packets to take. If all of your devices are wired, then nothing will be using the wireless connection because the "path cost" to travel the wireless connection is a lot higher than it is when travelling a wired connection. Sonos devices will still communicate over that wireless connection, because in the event that a device were to lose its wired connection, it would instantly learn that and have that device communicate wireless instead... but we're talking a miniscule amount of data (probably on the order of single digit kilobytes per minute).

Given that Sonos doesn't support systems with the wireless card disabled (the first thing they'll do if you have problems is tell you to turn it back on), it would be recommended to leave it turned on. As jgatie said, there are instructions out there that tell how to turn off the wireless card if desired. However since it's unsupported, it should be noted that there's nothing preventing them from removing that functionality in the future.

Also, since it is unsupported, Sonos has no responsibility to honor warranty support and/or replacement of a device which has been hacked in this way if such issues should arise.

It isn't a hack, simply a software setting you can make if you figure out where it is. Not supporting it is a lot different than voiding your warranty. Turning the radio back on is very simple and should be done before requesting Sonos support.

I'd not recommend doing it unless you are sure it is causing a problem with either Sonos or another WiFi device, and even then make sure it is actually solving the problem or turn the radios back on.

HI, Now With Sonos version 9.2 we are able to switch the wifi off for wired devices.
Good work Guys! Not only having more wifi for other things or less consumption, I Also was able to sell a system for someone who has problems with high frequency (electro-sensitive)!