How do I prevent a Note from being edited?

Question

Once a Note record is created in Salesforce, I would like to prevent that note from being edited by any user.

I tried this code but it is not working as I am still able to edit a note and the error message is not being thrown. Any ideas how else I can go about this?

trigger NoteTrigger on Note (before update, before delete) {
    if (trigger.isbefore && trigger.isupdate){
        NoteTriggerHandler.PreventNoteEdit(trigger.new, trigger.oldMap);
    }
  }

public without sharing class NoteTriggerHandler {
    public static void PreventNoteEdit(List<Note> nList, Map<Id, Note> oldMap) {
         for (Note n : nList){
             if(oldMap.get(n.id).Body != n.Body) {
                 n.addError('Once a note is created it cannot be edited or deleted.');
            }
        }
    }
}