The Washington Post plans to give up the Pulitzer Prize awarded its reporter Janet Cooke for a story about an eight-year-old drug addict. In a telegram to members of the Pulitzer Prize Foundation, the Post executive editor, Benjamin C. Bradlee, said: "It is with great sadness and regret that I inform you that Janet Cooke, the Washington Post reporter awarded the Pulitzer Prize for feature writing Monday, has determined that she cannot accept the award. She told Post editors early this morning that her story -- about an eight-year-old heroin addict -- was in fact a composite, that the quotes . . . were in fact fabricated , and that certain events . . . did not in fact happen." Miss Cooke offered her resignati on and it was accepted.