Comment by 3rdDeviation

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.