If the Bible really is the word of God, then it's just a matter of interpretation and not authority-- Paul would be the author of a divinely-inspired work, and his words would be canon. The only question would be if the JWs had the correct interpretation and application of his words. Many religions have rules for expelling members who cross certain lines. I think it's more a question of whether the JW shunning practice is defensible from a Biblical standpoint.
Edit: Journeyman beat me to the point!