Since a baptism isn't legally binding, it can't be annulled.
On a side note, it makes me really sad to see so many people discussing "what if" scenarios to skirt around, outlaw, or somehow force the organization to stop disfellowshipping and shunning people. It is such a cruel arrangement.
But even if they did away from disfellowshipping, they would still shun. It would become an unwritten rule that they would call a "conscience matter" so people didn't peg them as the cult they are. Even today, some shun people anyway even if they aren't disfellowshipped. (**raises hand**)
The only thing that will stop the madness is if people within the ranks decide to stop it. They are the ones with the power to stop shunning their families and they don't even know it.