Just start charging rent. 10-20% of the max buy-price of the house every month or you lose it. You can pre-pay 2 months in advance. Then also make it sellable for up to 110% of the buy price you initially paid for it. Maybe if we made it less enticing to keep them because there's 0 upkeep there'd be more people willing to admit "I don't actually use this" because it's costing them gil.