Sadly, this type of crime will continue because when you are dealing with people's emotions things can get very messy.
1. Choose a neutral location for exchanging the children for visitation.
2. Never be alone with the ex in a private place.
3. Keep your new relationships on the down low.
4. Even if you hate your ex, you once loved that person enough to marry him/her and they have people who love them and will suffer forever if they are taken away. You may not have any respect for your ex, but you should have some compassion for those who do love them dearly - like your children together.
5. Whatever misery you are currently suffering from the actions of an ex will pass and life will move forward. Hurt feelings and disappointment will pass.
Be happy, be kind - even if you don't feel either one - you will one day and it is worth the wait.