I rarely talk about anything beyond ground rules in a softball pregame. I would never discuss a specific rule unless it is a new rule (and it is the beginning of the season) or one of the teams has a specific question... then my answer would be specific.
The rules of the game are the rules. JUST CALL THE RUNNER OUT. No need for warnings or anything else. If what WMB does works for him, great. It does sound as though it could possibly create some expectations and some trouble.
How about this though... Dakota
EJECT THE COACH! If he is obviously cheating as you felt and 'making a travesty of the game...' "Time/Dead Ball, the runner is out for being off the base too soon, COACH YOU'RE GONE!"
Just a thought. I don't feel coaches and players are there to challenge the umpires or their abilities. You want to challenge an umpire you loose; you want to argue a call you loose. Same story. For me it would have been very tempting to eject the coach for encouraging rule violation.
From my viewpoint (hundreds of miles away and through a discussion board), it's like a coach telling the pitcher to throw at the batter, or sending a player in with illegal equipment, or to intentionally collide with someone. These kind of tactics are disgusting and worthy of penalty/punishment.
In my opinion and just a consideration. I would have really needed to be there in the situation to make such a decision.