For excluding the method one by one, you need to list it on the array,
[ 'controller/method_a' 'controller/method_b' 'controller/method_c' ];
There is a wild card for exclude 1 controller. For example you have log controller, and want to exclude all controllers not using CSRF token.
$config['csrf_exclude_uris'] = ["log.*+"];
.*+ is the wild card for the /method