Nathan Radley.
Jem stopped her.
That the knothole has been filled with cement. That's what was on Spark Notes.
Its in the subway, to your left
Jem and Scout discovered that someone had filled the knot hole with cement, sealing off the opening where gifts had previously been left for them by Boo Radley. This action by Nathan Radley signified the end of their secret communication with Boo.
Nathan Radley, Boo Radley's brother filled the knothole with cement. When Jem and Scout asked him why he was doing this he told them that his tree was dying and filling the knothole with cement would keep it from dying. Scout and Jem went and asked Atticus if he thought the tree looked like it was dying and he said it looked perfectly healthy.
He tells them the tree is dying.
Nathan or Arthur, I can't remember which, seals it with cement.
Nathan Radley fills the hole in the tree where gifts are placed with cement to stop Boo Radley from communicating with Scout and Jem.
The Radleys' tree getting filled with cement is described in Chapter 8 of "To Kill a Mockingbird" by Harper Lee. In this chapter, a mysterious figure fills the hole of the tree with cement, which upsets Jem and Scout.
In "To Kill a Mockingbird," according to Scout, Boo Radley lives in the Radley house.
Scout has stopped believing in childish superstitions that her, jem and dill all believed in. She now has things figured out about racism and the things that went on in the 1930's. Atticus greatly helped her with this task.
Trefoils/Shortbreads are the oldest Girl Scout cookies according to the box.