Joins both usb and wifi personal hotspot iphone
There is no built-in logic to specify an if/then decision to not join WiFi so the mac will join any WiFi SSSID that is detected and matches its list of auto-join networks.

You can set the service order by "secondary clicking" the gear at the bottom of the Network Preference Panes to Set Service Order... and make sure the WiFi is lower than the USB connection. It will still join, but no traffic will flow while the USB connection is viable.

Lastly, you could just make a new Network location called USB Only and turn off WiFi all together when you select that location on the mac. Changing network locations can be scripted as a service and AppleScript/Automater or FastScripts to effect or reverse the change you desire from a keyboard shortcut.