A fluid flow prevention device to be used with flexible conduits, and more particularly with toy balloons. The device is made of a resilient material and has a centrally and axially located normally closed slit through which a ring of a balloon's mouth may be inserted when the slit is forced open, whereby a neck of the balloon is maintained within the slit as the slit closes. After inflation of the balloon's body, the body and the ring are rotated in opposite directions, whereby the neck of the balloon is caused to twist. The resilient material of the device maintains closure of the slit whereby the wall defining the slit maintains the neck in its twisted position and prevents egress of fluid. In addition, the contractile force of the twisted neck draw the ring and the body of the balloon into frictional contact with the top and bottom surfaces of the device and further prevents untwisting of the neck and further prevents egress of fluid.