To nullify a baptism of a person who was a minor at the time requires this action:
As soon as the person reaches 18, the age of legality, they must stop attending meetings, and even claiming that they are a Jehovah's Witness. In this way the congregation cannont claim that the person affirmed their status as a JW after they reached the age of 18.
Doing this might give you a valid claim to nullify your baptism.
Probably will work as long as you don't cave in to their unreasonable threats and demands. What a bunch of turds.