I'm afraid you can't really just stop it immediately. It will get better with time though. In the meantime healthy distractions are probably your best shot. When I was in similar situation to yours I started spending pretty much any free time I had on two things: running and learning guitar. Half of the time I was doing it with tears in my eyes but it was still better than just getting lost in depressing thoughts. It took me like 3 months until the breakup wasn't dominating my thoughts everyday. It was hard AF but in this time I actually learned something, improved and rediscovered myself. Sport and music have become major parts of my life. Now, years later, I'm actually thankful for this breakup. I wouldn't be the person I am today if it didn't happen and I definitely prefer current myself to the past myself.
You just need to plow through the hard times. It WILL get better! I wish you all the best.
It's like a week since you went no contact if I gather correctly. That's still very early. In my case, for instance, I barely left my bed for the first 2 weeks. All the other stuff came a bit later. I don't really remember how much exactly. So yeah, unfortunately the answer is still just time.
I might have another suggestion as well. For me journaling was kinda helpful. My head was full of alternative scenarios, questions to ask and things to say to my ex. Since I couldn't do that, writing it down cleared my head a little bit. I guess don't do that if it will make you dwell on the breakup even more. But if that's all you think about anyway, then there's probably nothing to lose here.
Sending virtual hugs your way. Be strong!