You cannot disable this completely on an MX, unfortunately. We seem to have found a workaround, though:
Minimize the power of the 2.4 GHz band to make it virtually invisible by setting it to 1 dbm.
- Go to Security & SD WAN -> Wireless Settings
- Scroll all the way to the bottom of the page
- Change the Transmit Power for the 2.4 GHz band to 1 dbm
- Click Save Changes
Of course, don't do this if you have any devices that cannot use 5 GHz, because those devices will no longer be able to connect.