You're faced with an avalanche, flood, lightning storm, vice-principal, or other natural or unnatural disaster.
Negative reinforcement works, sort of.
If the parent let him into her bed, he stopped crying. To escape from the aversive crying. The only thing is, the person who nags becomes associated with the nagging,and then the naggee may try to escape from the person who nags, too. And the overall tone of the relationship slips, as nagging escalates. Highly aversive escape contingencies are almost always a problem.