I have a residential Family Care Home and the state requires that a business such as this have doors throughout the home that will not lock regardless of whether it has a locking mechanism. I could have gone out and purchased the ones that won't lock,but they cost almost $40 a piece (I needed 6), so I placed a magnetic metal strip over the door jamb to prevent the door from locking, and the door still closes perfectly. Ace, Home Depot, and Lowe's stock these strips, and most hardware stores will cut them to your specifications for less than $2 a piece. This way if you decide to sell the home, you simply remove the strips.
Doing this saved me a lot of money, and it works!