The Guinn case (the one mentioned by Gio in the OP) is not precedent for legal action for WTBTS-style shunning. In fact, I took a class in law school where the Guinn case and Paul v. Watchtower (discussed elsewhere on this site; an unsuccessful lawsuit against the WTBTS for shunning that made it to a U.S. Court of Appeals) were discussed in class on the same day to juxtapose which church disciplinary actions are protected by the Constitution under U.S. law and which aren't.
The Guinn case is different because after the individual withdrew membership from her church, the church continued to treat her like a member. After her withdrawal, they (1) announced her sins from the pulpit; (2) told church members to find and confront her to try to get her to "repent"; and (3) sent letters describing her "sins" to neighboring churches. That was the basis for liability - the church violated Guinn's First Amendment right not to participate in a religious organization. It was not merely shunning that amounted to actionable conduct. The same would obviously not apply to WTBTS policy. If you withdraw, they will announce merely that you are no longer a member, and they will no longer treat you like a member, with all that we know comes along with that.