Opening a door with a credit card is possible because the thin material of the card is able to slip under the door bolt and simply slide it out of the way. This presents a problem for homeowners because of outside threats, but it can also be problematic inside the house if a family member, guest or tenant is getting in doors where they shouldn't. To prevent the possibility that an interior door can be opened with a credit card, some extra security measures must be taken.
Replace the trim around your door frame. A thick trim that covers the strike plate (the metal part that's on the door frame) will prevent someone from slipping a credit card between the bolt and the plate. A door that swings inward will not be affected by a thicker or overlapping trim.
Install a door latch guard on the door if it swings outward. This is a metal plate that screws in next to the door handle and covers the strike plate so a credit card cannot get to the bolt.
Install a dead bolt that can be opened only with a key. A deadbolt that slides about an inch straight into the door jamb cannot be opened with a card, unlike a regular door handle that can be pushed out of the way.