Open AirPort Utility
Click on the picture of the AirPort Express, then click Edit
Click the Wireless tab at the top of the window
Click the Wireless Options button
Change the Radio Mode to 802.11n only 5 GHz
Click Update to save the changes
Only 5 GHz capable devices will be able to connect to the AirPort Express, since it is no longer providing a 2.4 GHz signal. In addition, 5 GHz signals are much weaker than 2.4 GHz signals, so you will almost need to have line-of-sight between a wireless device and the AirPort Express for good connection quality.