Stop the ends of rope from untwisting using the whip knot which is basically binding the rope with a whipping twine. See explanation below the drawing.
(1) Lay the twine on the rope, making a loop.
(2) Start wrapping the twine around the rope, making sure to entrap the loop.
(3) Continue wrapping the twine around the rope and then thread the twine through the end of the loop.
(4) Pull down end (a) until the loop is pulled into the wrapped twine and then cut off excess twine on both ends (a and b).