Help replacing corroded bathtub spout

Hello, a few weeks ago, the spout to my bathtub fell off and I couldn't seem to screw it back on. I took a look and the threads of the galvanized outlet have essentially rusted off. I took a pipe wrench to it and it didn't budge one bit. I'm afraid of forcing it in case I break the pipe in the wall. Does anyone have any suggestions of what I can do to replace it? Thanks. Attachment 42435 (http://www.doityourself.com/forum/attachments/plumbing-piping/42435-help-replacing-corroded-bathtub-spout-imag4029.jpg) Attachment 42436 (http://www.doityourself.com/forum/attachments/plumbing-piping/42436-help-replacing-corroded-bathtub-spout-imag4031.jpg)