What is the expiation for a man who spread rumors about a female colleague at work, causing her severe harm, and refused to clear himself of what he said about her?

Harming Muslims and accusing them of things they have not done is a form of transgression and injustice for which Allah has threatened the doer with immediate and deferred punishment. Allah Almighty says: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin." And whoever speaks ill of a believer that which is not in him, Allah will house him in the mire of Khabaal. There is no sin more deserving that Allah hastens the punishment for its perpetrator in this world, in addition to what He reserves for him in the Hereafter, than transgression and severing ties of kinship. It is your duty to this woman to retract your accusations against her, exonerate her from what you have falsely attributed to her, and remove the harm you have inflicted upon her. Your repentance will not be complete except by doing this, along with regretting what you have committed and resolving not to return to such an act.