Teenage Open Adoption Issues

We adopted my teenage son a year ago. He has been with us two years. He chose to leave the situation and asked us to adopt him. At first we only had power of attorney, so good boundaries couldn't be set, because she would just take him away, so he went over there when she wanted (even though it wasn't safe), and called when she felt like it (once a week or once a month). Since then, she has accused me of having sexual relations with my son, that I have stolen him, i have had to call the cops on her, we have called CPS on her for her two remaining kids numerous times. She just recently got arrested for a DUI. She calls when she wants and promises things that won't happen to my son. She says that they will get together (we sit in a park next door when they do - she doesn't know we do that) and then she doesn't go through with it. My son has been diagnosed with PTSD and major depression. He is 16. She won't let him call me mom in front of her, she gets drunk and calls and cusses me out. It hasn't happened in a while, but overall has happened too many times to count. I am in the end of my Masters Degree. I had to take a test to get my degree. She called on the day of the test and started in on how she wants to see him and causing drama. I failed my test. I have been given another opportunity to take the test. How do I eliminate this stress? Even when she is good, I'm stressed because I know it will go bad. I already blocked her from being able to call my son's cell phone because it was too stressful with her calling drunk and yelling at him. Do I block her from my phone? He wants a relationship with her and his siblings, how do I find a good medium? Have her call my husband's phone? Help please.