You could go the other direction -increasing the borrowing cost of purchasing a home for investment to ensure that borrowers with less capital can still afford homes in the same market.

As a hypothetical, you could tax their purchase at the same rate at which they're borrowing and use the funds to back loan guarantees for new/lower income purchasers.

The point is to impair the ROI for multi-home purchasers without limiting upside in the market.

NINJA loans were loans for people with no income and no jobs. What I’m proposing is cheaper access to cash for first time homebuyers. You’d still have to be credit worthy.