Final code for myCRED Tutorial - http://mycred.me/tutorial/block-users-with-negative-balances/

block-negative-balance-access

/**
 * Block Negative Balances
 * Stop users from logging into your website if they have a negative
 * myCRED balance.
 * @version 1.0
 */
add_filter( 'authenticate', 'mycred_block_negative_balances', 990, 3 );
function mycred_block_negative_balances( $user, $username, $password ) {

	// Make sure myCRED is installed
	if ( ! defined( 'myCRED_VERSION' ) ) return $user;

	// The point type we want to use
	$type = 'mycred_default';

	// Load myCRED
	$mycred  = mycred( $type );
	$account = get_user_by( 'login', $username );

	if ( isset( $account->ID ) ) {

		if ( ! $mycred->exclude_user( $account->ID ) ) {

			if ( $mycred->get_users_balance( $account->ID, $type ) < 0 )
				return new WP_Error( 'mycred_negative', __( 'Account Closed' ) );

		}

	}

	return $user;

}