C#

NodeModel folder = (from f in Folders
                    where f.Id == model.Id
                    select f).FirstOrDefault();
Folders.Remove(folder); 

void Remove<T>(IList<T> list, T node) where T:INested<T> {
 if(list == null) return;  // in case you use ChildNodes=null to specify no children
 for(int i = list.Count; i >= 0; i--){
  if(list[i] == node) list.RemoveAt(i);
  else Remove(list[i].ChildNodes, node);
 }
}

interface INested<T> { IList<T> ChildNodes; }

interface INested<T>
{
    IList<T> ChildNodes { get; set; }
}

if (list[i] == node) list.RemoveAt(i);

// Base interfaces for a node
public interface INode<T> 
    where T : INode<T> // A child is the same class as the parent
{
    Guid Id { get; set; }
    List<T> Children { get; set; }
}

// Static class containing Extension Methods
public static class NodeExtensions
{
    // Recursively removes a node by its id
    public static void Remove<T>(this INode<T> node, Guid id)
        where T : INode<T>
    {
        // List that will contains the nodes to be removed
        // Can't remove nodes directly in the foreach loop
        // (will throw "Collection was modified; enumeration operation may not execute.")
        List<T> toRemove = new List<T>();

        if (node.Children == null)
            return; // No childrens, exit

        foreach (var child in node.Children)
        {
            if (child.Id == id)
                toRemove.Add(child); // The node has to be removed
            else
                child.Remove(id); // Recursion here
        }

        // Removes all found nodes
        foreach (var child in toRemove)
            node.Children.Remove(child);
    }
}

Folders.Remove(Folders.Find(f => f.id == model.id));