Clients who are looking for last year's AGI (or depreciation schedules for that matter) have already decided to go elsewhere. I would have no qualms about giving them what they want and saying goodbye. Why make it hard when the relationship seems to be broken? They want to use a preparer who is closer to where they live, who their friend uses, whose price is perceived to be lower, or who will let them deduct the suits they buy for work that you would never allow them to write off.
Sometime they come back with their tail between their legs. Sometimes when they try to come back you gleefully tell them you're not accepting new clients. Most often you never hear from them again. I would never do anything to force them to stay. What's the point?