Slander, libel, and defamation are civil issues. If you have proof, you can file a civil suit. Trick is, you have to prove that it cost you money. As far as harassment, look up your local harassment laws. You may be able to get an RO.

One time i was being harassed via text message by my exh. This lasted for months, and i was literally getting 20-30 texts a day. It was breakin me down. I was so depressed, so exhausted and scared. I was literally starting to fear for my life. The way he went on and on and on text after text, the anger, the frustration, the unpredictability. It was frightening!
I finally went to the police only to be in tears! The dispatcher was so cold, condensending, just awful. She spoke to me in that "oh geez, we got a stupid whiney woman here complainin cause her ex sent her some texts" tone. She kept askin if he threatened me? Has he directly threatened to harm you? Blah blah. No, he hadnt but you cant tell me that what he isnt doing is harassing.
Well, if he hasnt threatened you.....
So i go to the police station. The lady at the desk same attitude!!!! (Same lady? could be) i am BAWLING!!!!! Please please, cant someone help me? I cant believe, he can text me saying all these awful things to me at all hours and i cannot do anything!!!? (Yes i saved all the texts)
The desk lady finally went to get an officer. (To shut me up) he was much kinder, but still was like "we cant do anything. He hasnt threatened you. You guys share custody, he has to contact you. Just ignore the texts, blah blah blah"
I looked dead at that officer, with an eerie seriousness and tears streaking down my face and i said.....
"I come for help. A man who is very very angry, very bitter, and very spiteful, is texting me mean ugly hurtful spiteful angry words right now. He texts me ANY time he wishes. While i work, while i sleep. I can do NOTHING to stop him. Im powerless and scared. No he has never threatened me. But his persistence in his anger is enough for me to feel threatened."
The officer apologized again, "nothin we can do, blah blah"
I started to gather my stuff, tears still flowing. I apologized for taking his time and i said, " if you cannot help me, then no one can. Harassing is illegal, and this is a classic definition of harassment. Its hopeless." He was trying to console me, "maam, now come on maam, blah blah" i said, "no sir, i get it. And so do all those DEAD girlfriends, wives, ex wives, ex girlfriends, that went for help and was told there was nothing any one can do cause he didnt threaten her. Or how about these men who just snap and kill these women? People who know these guys say they wouldve never thought that guy would be capable of killing his girl.
I left. The officer came to my door with a case number, and a paper stating what the meaning of harassment was. Classic example like i said. He told me he looked more into it and will be talking to my ex. And since there is a case number, then if he texts me with more harassing stuff, i can pursue more.

So if you feel your being harassed, really really feel it. Then DO IT! call the cops, file charges. I was bullied in my marriage by him, and he tries to bully me now. No one should be harassed!!!!
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If she is threatening more than 1 person it can be construed as terroristic threats.
However if this is just you, record record record. Maybe make an appointment with an attorney to see if you can file a 'cease and desist' order. Or maybe this is her way of annoying you and getting your attention just to keep you unhappy and away form her kids.

As for the kids, in reading your profile it sounds like you have enough love for everyone plus some. There is nothing wrong with loving your stepkids but sometimes the BMS see this as a HUGE threat. You may be being seen as trying to make a new family for her kids and taking them away from her. An insecure woman may see it like this.
Iam not judging you because there is a little place in my heart for my 2 adult stepsons. I love their characters and the way they are developing (at last!) into acceptable members of society. I never thought I could feel this way about someon elses kids. DH and I will never have our own children and these kids are azn extension of him..... and their ice queen mother. And I have to remember that ALL THE TIME.

Why not change your number? your x can either call you on a landline or he can send you an email. He shouldn't have your cell anyway