You are dealing with an aged CHILD. You need to react like an adult. She is a school yard bully calling names and throwing tantrums. Since the ex is someone else's child, you do what you would with any child who isn't yours but you find yourself having to deal with them... like one of your friend's kids or your child's friend.
1. Ignore low level poor behavior pretend it didn't happen
2. When they are in a snit don't play with them
3. Any time you MUST interact with them, do so civilly and and minimally.
You would never let some neighborhood child's ranting and raving make any difference in how you handle your own household; you would never let their opinion matter. You would laugh it off because it is ridiculous for them to think they have any say. Same situation here.
Do not react to ANYTHING she says unless it is specificaly about the well-being of the kids. You can take a little name calling and malicious gossip. It does not change who you are or how well you parent. Teach her how to treat you. When she behaves, reward her with a little more contact. When she behaves poorly, restrict her interaction even more. If she makes allegations to the police or to the courts you address them directly with the courts or the police.
When with the kids, you address lies as misconceptions. Teach them tools to deal with lies in every day life. (e.g., "I don't know where little Jimmy got the idea that we eat dog food... we don't have a dog and you can see for yourself that there isn't any in the cupboards. Have you ever seen us eat dog food? Little Jimmy must be mistaken." or "I don't know how your mom came to think I hate her. Have you ever heard me say that I hate your mother? I don't feel that way." Then make a joke about how their mom must have crawled into your head and read your mind to know what you feel? No? hmmm what do they think? "I think she must be mistaken.")
You need to change every phone number except one. That should be your husband's cell - easily turned off at night. Have him write her a a letter stating that the only number available is his number and you will only respond to calls regarding the kids' needs, schedules and emergencies. Then do it, change all the rest. Let her messages go to voicemail and then check it immediately - respond to whatever is necessary and maturely ignore her childish tantrums. You as step-mom should only respond if it is an emergency and he is otherwise indisposed. (e.g., Janie is at the hospital? Which one? what do her doctors say about her condition? We'll be on the way in the next 15 minutes.)
Finally, stop renegotiating the
parenting plan. stick to it 100% - no trading, no changes, no nothing. You want a judge (usually the same one who signed off on the order in the first place) to take you seriously, then respect his order. The only exceptions should be for emergencies, a death in the family and other truly serious matters. If the kids have a school function on your time, you do the right thing and take them. You two need to get on the same page and stop playing her game.