The onus is to prove that shunning IS scriptural, not the other way around.
That having been said,
http://www.jwfacts.com/watchtower/disfellowship-shunning.php
http://www.jwfacts.com/watchtower/disfellowship-shunning.php#unscriptural
http://www.jwfacts.com/watchtower/disfellowship-shunning.php#bible
http://www.jwfacts.com/watchtower/disfellowship-shunning.php#history
As JWFacts notes, shunning was condemned by WTS in 1947, but then copied RC in 1952.
Also, in WT August 1, 1974, there was a fairly balanced article on not being fanatical shunners.
Divine Mercy Points the Way Back for Erring Ones
Maintaining a Balanced Viewpoint Toward Disfellowshiped Ones
The Ray Franz' and others' "revolt" in Brooklyn led to the 1981 position that has been vigourously applied since.