If the voter has the ballot, ask the voter to hand it to you, write ABSENTEE SPOILED on it and then you can process the voter as a routine voter on the Poll Pad. You will need to provide a password to update them on the Poll Pad. The spoiled ballot will go in Envelope #4.
If the voter does not have the ballot, you can have them vote provisionally. Provide them a GREEN Provisional Envelope, Provisional Notice and a ballot. Make sure the envelope is filled out completely and the reason code #4 box is checked. Samples of these forms can be found on pages 7.5 and 7.6 of the Chief's Notebook.