NotebookDelete[notebook] deletes the current selection corresponding to the specified notebook object.
Using NotebookDelete in the kernel is equivalent to using the Clear command in the front end.
After NotebookDelete, the current selection becomes an insertion point at the position of the deleted material.
notebook must be a NotebookObject, as returned by NotebookOpen, etc.
See The Mathematica Book: Section 2.10.3.
See also: NotebookRead, NotebookWrite.