Let it Go
We've all been there. The relationship is over, but not everyone is letting go. How do we release what's no longer working?
"I want my ex-husband to break away from me and leave me alone."
I'm not sure of the details of your situation, so there are probably other tangible steps that would be helpful (Restraining order? Social media blocking?), but I'll just approach this from an energetic and spiritual perspective, since that's probably the reason you asked a witch.
I'll start by saying that I don't personally work magic on others without their permission, nor do I advocate for it, so my suggestions will be focused on what you can do for and with yourself. There are plenty of witches who will hex or bind another and if that's your path, great, but you'll have to look elsewhere. I am a fan of consent and self empowerment, so that's where I'm coming from.
The first step in disconnection is a strong desire and intention. Speaking from experience, if there is any part of you that still wants to hold on, any magical steps you take won't be effective. Write down clearly the outcome you want. For example, "My ex husband has moved on and we are separate and disentangled from each other wholly, for the highest good of all involved" or something like that.
Try to approach your magic from a place of love and forgiveness (this doesn't mean you condone any actions or even forgive them, but if you can get yourself into a place with this frequency it helps), and use gratitude to get there. Reflect on the reasons you are thankful for this person and review the lessons you've learned and gifts you've received from the relationship (for example, I've learned to value myself more and am grateful for our children, etc) I recommend this because then you are operating from a place of empowerment, not victimhood, which makes our magic much stronger. It's also easier to let go when we can take stock of everything we've gained in our process.
Once these steps are completed, there are a few ways it could go. I suggest not only a cord cutting, but also cord detachment. Using a match, move around your body, guided by your intuition, imagining all the places the cord that binds you has wrapped around your internal and external body. Wind it all up onto the match and then set fire to it, letting the fire draw out any pieces you may have missed. Alternatively, or in conjunction, you can do a visualization. Call all of your energy back to yourself, from everywhere/everyone you've ever left it or given it away. Do this with love. Welcome your energy back and let it circulate through your body. As it moves in you, let it push out anything that's not yours. There simply isn't space. Imagine foreign energy going either back to who it belongs to or into the earth to be composted. Repeat these steps until you feel clear and energized.
Once this part feels effectively done, finish by reinforcing your energetic field. Imagine your aura and search for places that may feel weak or damaged. Fill them with light and visualize everything being repaired and strengthened. Set clear intentions to keep your energy body clear and autonomous. Anchor your work and intention into your body with movement, breath, song. Create an altar to your sovereign self, perhaps including pictures you love with only you, flowers, jewelry you bought yourself, candles…
You may also consider rewriting your agreements. Write down the new contract you wish to have with this person (when you were married, your vows were a contract. You've since outgrown those particular agreements). The new contract can be simple, ie "(insert names) are separate from each other and all contracts we made previously are null and void." Then sign your name and his and set fire to it, to send it up into the ether.
You may need to continue these steps for awhile, especially if he reaches out (energetically or actually).
I don’t know the depth of your attachment but marriage tends to create pretty strong contracts and entanglements, so be patient with the process. Hire a professional if you feel you can't do it on your own. Hopefully you’ll soon find that as long as there isn’t any energetic connection between you, he’ll eventually lose interest and turn towards something else.