Please note that this is how I figured out how to disable the 2.4GHz channel on the AirPort Express. Please Proceed At Your Own Risk:
Setting your Wireless Router (AirPort Express) to only use the 5GHz Band with the AirPort Utility App
Connect to the router via the AirPort Utility
You will see the router at the bottom of the page
Select it and a dialog box opens.
You’ll probably have to log in at this point if you’ve password protected your Router
Select Edit at the top right of this box
The next window opens, you see on the left side:
Base Station
Network
Etc
Look at the bottom right for Advanced
Select Advanced.
Next window you see the following on the left:
DHCP and NAT etc. Look for WiFi Settings
Select WiFi Settings.
This next window you see, in my case,
5GHz Name
Country
Radio Mode
Radio Channel
Select Radio Channel
Now you’ll see:
Automatic Channel and underneath 5GHz. On the right side is the slider switch, in my case showing green which means it’s enabled.
Underneath this group is the Automatic Channel for 2.4GHz
This is the one you want:
If the right side slider switch is on, showing green, the 2.4GHz channel is operating. If you only want to use the 5GHz channel, slide this switch to the left.
I hope this helps anyone who is interested?
From my limited knowledge this completely disables the 2.4GHz channel so that you are operating only on the 5GHz band. The more learned here may chime in if I'm out in left field with this one and please feel free to correct if I've lead anyone astray. This method did work for me, in the way I described it.