For anyone that wants to know or gets here via Google.
We've had the same issue with a M425dw.
We've managed to kill the SSID broadcast by doing the following.
Go to the Networks tab and click on the Wireless Direct thingy in the left menu, change the SSID to anything you wan't and then select Off in the dropdown menu above the SSID field.
Disclaimer: printer language is set to Dutch, so I hope I have the Tab and Menu name's correct