Using backbone, can I prevent the nonce from being set?

Yes, the nonce is by default always being sent via the X-WP-Nonce header – see the source here and here on GitHub.

wp.api.WPApiBaseModel.prototype.sync and wp.api.WPApiBaseCollection.prototype.sync can technically be extended or modified, but I would instead disable the nonce header like so, i.e. using <Collection or Model object>.endpointModel:

• Collection example:

  const Posts = new wp.api.collections.Posts();
  
  // Remove the nonce to disable the X-WP-Nonce header.
  Posts.endpointModel.set( 'nonce', '' );
  //Posts.endpointModel.unset( 'nonce' ); // This also works.
  
  Posts.fetch( {
      data: { per_page: 2 },
  } ).done(
      data => console.log( data )
  );
  

• Model example:

  const Post = new wp.api.models.Post( { id: 1 } );
  
  // Remove the nonce to disable the X-WP-Nonce header.
  Post.endpointModel.set( 'nonce', '' );
  //Post.endpointModel.unset( 'nonce' ); // This also works.
  
  Post.fetch().done(
      data => console.log( data )
  );