Using my library undo.js, you can disable undo and redo actions (not just from shortcuts) using the following code:

undo.observe(document.body, {
    preventDefault:true
});

You can then capture requests from the user for undo and redo actions (using a shortcut or the browser menu) using:

myeditor.addEventListener("undo", function(e){
    // whatever...
});
myeditor.addEventListener("redo", function(e){
    // whatever...
});