Who finally stops the boy?

The Provost stops him by removing the grain the Virgin had placed on his tongue.

"Before the child was buried, holy water was sprinkled onto him, and he began to speak. The abbot of the abbey questioned him as to how he could sing, and the child answered that the Virgin Mary had placed a grain on his tongue that allowed him to speak. The abbot took this grain from his tongue, allowing him to die, and finally pass on to heaven. The child was buried in a marble tomb as a martyr, and the tale ends with a lament for the young child, but also for “Hugh of Lyncoln” (a real child martyr, allegedly slain by Jews in Chaucer’s day)."