How do I hook into the Wordpress login system to stop some users programmatically?

Question

I am working on a Wordpress based portal which integrates with a custom-made e-commerce. The e-commerce serves also as a \'control panel\': all the roles are set up there.

Answer 1

There were a few issues with mjangda answer so I'm posting a version that works with WordPress 3.2

The main issues were with the return statement. He should be returning a WP_User Object. The other issue was with the priority not being high enough.

add_filter('authenticate', 'check_login', 100, 3);
function check_login($user, $username, $password) {
    // this filter is called on the log in page
    // make sure we have a username before we move forward
    if (!empty($username)) {
        $user_data = $user->data;

        if (/* check to see if user is allowed */) {
          // stop login
          return null;
        }
        else {
            return $user;
        }
    }

    return $user;
}