I had a similar experience while learning to fly, I broke the wings on my trainer (royal air 40-t). I used a dremel tool with a cut off wheel to cut the joiner out as far as it would reach, then I just scraped away at it with a knife, and saw blade...took a while but it came out.

After making a new joiner I glued repaired and glued the wings together. You may want to add fiberglass cloth to the joint to further strengthen it.