Several factors must be considered.
1) Einstein proved that spacetime is a fabric, upon which matter resides and in turn that matter warps the fabric of spacetime creating gravity. Since all matter must remain on the fabric of spacetime nothing can be flung off the edge of th Universe since the Universe IS spacetime as well. In order for matter to be lost at the edge of the Universe then the fabric of spacetime would have to be torn away.
2) Where there is nothing, nothing can exist. Many think of what is beyond the edge of the Universe as "empty space" or a "vacuum in space". Neither statement is true. Beyond the edge of the Universe there is simply nothing. Nothing empty, nothing void, just nothing. When you reach the edge of spacetime you reach the edge of the created Universe. When spacetime ends at the edge of the Universe all basis for the existance of matter ends.
Therefore without the fabric of spacetime and with the absence of all that matter needs to exist matter cannot exist or flung free of the edge of the Universe.