How to disable WIFI extender in Airport Express but leave ethernet working? I am using an Airport Express as an extender for the purposes of using its ethernet cable for a network device. In this setup, The Airport Express also the behavior of extending my network from my Airport Extreme. However this is causing me problems with our Apple TV since the Airport Express WIFI spec is much slower than the Airport Extreme. 

We don't need to extend the wireless network. Is there a way to turn off wireless extension and leave the ethernet connected?
Yes you can do this in Airport Utility. Unfortunately I don’t have it open in front of me so I can’t show you a screenshot of where in the settings to do it but if you were able to configure it  in the first place then just go back to the same place and there should be an option to disable wireless entirely. 

If you have an older Airport Express then you’ll need Airport Utility 5.6 running on an older version of OS X. If you don’t have that handy, you’ll have to make it happen by installing an older version of OS X in a virtual machine.

Hope this helps.