Check to see if you have multiple default routes assigned. You very well might have one for each network. If the default route for the IP address you are using is only for that interface, you can delete that route and then the interface.
A more simple thing to do is just use option 10 from the CFGTCP screen to stop that interface. I would go in and change the interface so it does not come active at IPL (option 2 keyword: AUTOSTART. Set it to *NO)