Wait until dad goes to the bathroom, then cut the strings, then run away. When he comes looking act all sorry, even work up a tear or two to show dad how bad you feel that he was sold a defective banjo. Something like this:
DAD..."Look what happened to my banjo! Do you know anything about this?"
YOU... "No dad. OH MY GOD, what happened? Dad, those bastards sold you a defective banjo (sniff, sniff). I saw a show about the banjo scammers on 60 minutes (small sobbing cry). You had better return that right away. Besides dad, you're wayyy to intelligent and talented to play the banjo. I think a man of your abilities needs to do something cool, like model ship building. You would rock at that!"