The code above is the only way without changing the files. Hooks can be tricky, the order of execution is important.
Maybe using the hooks to the mce as well could work:
remove_action('admin_head', 'wpdreams_asp_add_mce_button');
remove_action('admin_head', 'wpdreams_asp_mce_generate_variable');
remove_filter( 'mce_external_plugins', 'wpdreams_asp_add_tinymce_plugin' );
remove_filter( 'mce_buttons', 'wpdreams_asp_register_mce_button' );