How can I prevent Desktop Users from connecting to new wireless networks?

You can kill nm-applet (the thing in the bar at the top that let's you choose the network settings), and use iwconfig to choose the wireless network, like this (assuming your wireless NIC is wlan0):

$ sudo iwlist wlan0 scan

This will display all available wireless networks.

$ sudo iwconfig wlan0 essid "MyWirelessNetwork"

This will try to connect to the wireless network with SSID "MyWirelessNetwork"

$ sudo dhclient wlan0

This will get an IP from the DHCP server.

Since the above commands require sudo powers, regular users will not be able to change the wireless settings.

If you choose, you could remove the nm-applet all together by removing the package network-manager-gnome