A twist off tamper-proof fastener having an elongate flexible strap defining a plurality of parallel adjacent transverse teeth in opposing planar surfaces. First end portion of elongate strap is tapered to a rounded tip for insertion through a “T” slot defined in a dimensionally larger head portion carried at opposing end of the elongate strap. A fixed tooth and plural angulated teeth carried by a flexing arm irremovably engage with the teeth defined in the elongate strap. The head portion is elliptic with opposing spaced apart rounded ends and opposing planar sides to facilitate grasping and twisting by a user, and for identifying indicia. A circumferentially extending notch is defined in the elongate strap immediately adjacent the head portion. Twisting the head portion creates shearing forces that are focused at the circumferential notch breaking the fastener and allowing removal without tools.