How to disable WIFI extender in Airport Express but leave ethernet working?
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.