if it was me...
I would try heating the remain pipe with a torch then putting ice on the inside..
then try to unscrew the remaining copper pipe..

try cutting the copper pipe with a saw. gently. more to score it then cut thru..
then pry the copper towards the insides..then unscrew.

if that fails..open the wall up and replace the pipe..

-dkenny

Buy an "internal pipe wrench"....or open the wall and redo it.

Video clip below. Watch from the 1:00 mark.

Here is a link that might be useful: internal pipe wrench

This post was edited by homebound on Sun, Jul 7, 13 at 22:19