Disable auto-save and post revisions from inside a theme or plugin

You mentioned:

Is there a way, to do that from within a plugin or a themes functions.php?

You may want to try the following code. Just place it inside your theme’s functions.php file. Using it, you do NOT have to alter the wp-config.php file.

define('WP_POST_REVISIONS', false);
function disable_autosave() {
    wp_deregister_script('autosave');
}
add_action('wp_print_scripts', 'disable_autosave');