My Girlfriend's son has been signed by a Premiership football club. They have zero tolerance on any misbehaviour by players OR parents. This is obviously a pretty high level (even for a 10 year old) but they don't mess about. No second chances, no excuses, toe the line or you're out. I asked the coach about it and he was pretty open about the thinking behind it. Any behaviour like that has a negative impact on everyone there, it doesn't matter how good the kid might be, he's making his team-mates play worse....if the parents can't control themselves then they blow it for their kid. Simple. He reckons that even at that age the kids have got to 'man up' and if that means them having a pop at their parents then so be it!
Stop messing about Leonie, you've said yourself that this guy's behaviour is having a negative influence on the other team members. Ban him from the training sessions, and get your board to back you up. If you don't do something it will only get worse. No one player is more important than the rest of the side.