Can you take down the wireless with:
ifconfig wlan0 down
then bring up your wmx0?
Are you using ceni? If so, you may be able to set the network interfaces in the /etc/network/interfaces either manually or with ceni - I don't know if ceni has an option not to bring up the interface. On reboot you may then be set.
Actually, now that I think about it some more, assign your wireless an ip address for a network you're not using in your shop/home. Then reboot so wlan0 is not active. You may be able to use ceni or wicd to bring up wmx0.
Something tells me you're going to have to use ceni and the /etc/network/interfaces file to make this work - some old school.