EOM, as problem addict said, the conduct of the WTS does not meet the legal definition of libel/slander/defamation the U.S. Their characterization of those who leave is morally reprehensible, but it isn't defamation. Defamation has to be more specific. The adage goes that if you defame a large group, you defame nobody.
What ABibleStudent said is probably the most realistic thing you can hope for - some alteration of tax exemption or tax deduction laws. He is right that the 1st Amendment may not be violated by that kind of approach, depending how it's implemented. At the same time I don't know whether it's feasible politically or practically to let some religions have tax breaks and others not to have them in the foreseeable future.