Abstract: A device insertable into a fluid pipe to obstruct free passage of a solid member therethrough. The invention is particularly useful to prevent the siphoning of gasoline from an automobile tank. The improved device comprises an assembly of a conventional coil spring and a flexible tape extending through a length of the spring from one end, extending outwardly of the spring and then connected to convolution of the spring at a spaced place along the spring, the tape then being doubled back and extending through the eye in a head at one end of the tape. The assembly is inserted within the filler pipe of the gasoline tank and a pulling force is applied to a free part of the tape which is located outside of the filler pipe, while the upper end of the spring is held against substantial movement. The result is a doubling-up of the spring so that it crisscrosses and obstructs passage of a siphoning tube to the gasoline within the tank.