What do i do about my ex husband violating the divorce papers?

My ex husband and his mother have time and again violated the agreements made in our divorce papers. We have a parenting plan that states simply that neither of us will receive any type of slander or verbal abuse in front of the children. The last three times he has been the one to drop them off he starts arguments in the front yard with the kids watching through the windows. They have made bogus allegations and had DHS at my house and he is threatening me with taking the kids away. I am remarried and living in a stable home with my husband. He has never been the one to get the kids for visitation, his mother gets them on the weekends that are supposed to be his. He is a deadbeat dad who recently joined the military because now he has 3 kids to pay child support on. What do I do about the verbal abuse and his blatant disrespect for the parenting plan?