Deactivating a contact for a certain timeframe
One option would be to add an Activity on her record with a Scheduled Date for 18 months time, and assign it to a contact that has an email address that will still be read in 18 months. that way you could change her email address to the one she wants using, or set her Comms Prefs to 'do not email', and then in 18 months they should receive a reminder to undo things so she gets the emails again.