Any way to control every instantiated prefab?

I want to make it so that when the player dies, all instantiated prefabs stop moving. Is there any easy way to do this?

Use this as a base class for a script attached to the instantiated objects:

using System.Collections.Generic;
using UnityEngine;

public class MonoCollection<T> : MonoBehaviour where T : MonoCollection<T>
{
    public static List<T> all = new List<T>();
	
	protected virtual void OnEnableEx()
	{
	}
	
	protected virtual void OnDisableEx()
	{
	}
	
    protected  void OnEnable()
    {
        all.Add(this as T);
		OnEnableEx();
    }
	
    protected  void OnDisable()
    {
        all.Remove(this as T);
		OnDisableEx();
    }

 }

E.g.

   public class Finder : MonoCollection<Finder>
   {
   }

Now you can have a list of everything with that script available using Finder.all:

    foreach(var f in Finder.all) {
         ...
    }

how about using the GameObject.FindGameObjectsWithTag method?

So you can tag all the prefabs that are supposed to stop, with the same tag. And then when your player dies you can run the method above and make sure they all stop their movement.