It would actually be easier to just set massive taxes for anyone that owns more than 2 houses. I don't begrudge anyone who owns a house and a vacation house but fuck anyone who just owns rental properties, and if you can afford to buy a third house then you can afford to pay a ton more in taxes to keep it. The more houses you own the higher the taxes go, just make it untenable for people to own rental properties and the rest sorts itself out.