Play after the whistle

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At this for two years and I bounce between 7/8th and JV assignments. 7/8th are more prone to this issue because of more held ball situations but very frequently the players continue to wrestle for possession of the ball after the whistle to stop play has sounded. This botheres me because that type of competition gets the emotions going and is a stimulus for other rough play. My solution:
Point of emphasis during Captain's pre-game meeting
Point of emphasis during Coaches' pre-game conference
Warning to all players during first held ball.

Unfortunately, none of the above worked and at the next wrestling match after the whistle I whistled both players for Double Technical-unsporting conduct.

1000 yard stares from both players and both coaches but it seemed to work for that game.

If players haven't been coached accordingly, the pre-game warning may have been/be a waste of time. Any thoughts on how this situation might be better prevented or controlled?