You can sue, anyone can sue. You could show the judge what such an announcement implies (esp. with the mentally diseased comment) and as poster said, simply dragging each individual through court is in itself expensive (as poster says, 8-12k in lawyers is not unusual).
This is civil court, not criminal nor is this a free speech issue. You just have to prove what you say beyond the preponderance of the evidence (meaning it has to pass the test of "probably" being true). WTBTS doesn't want any precedent in civil court in regards to that.