Invalidate username if it contains @ symbol

In a multi-site installation it is 'wpmu_validate_user_signup':

add_filter( 'wpmu_validate_user_signup', 'wpse_77904_mu_no_at_in_username' );

function wpse_77904_mu_no_at_in_username( $user )
{
    if ( FALSE !== strpos( $user['user_name'], '@' ) )
        $user['errors']->add('user_name',  __( 'That username is not allowed.' ) );

    return $user;
}

On single-site it is 'validate_username':

add_filter( 'validate_username', 'wpse_77904_no_at_in_username', 10, 2 );

function wpse_77904_no_at_in_username( $valid, $username )
{
    if ( FALSE !== strpos( $username, '@' ) )
        return FALSE;

    return $valid;
}

Caveat: both not tested. 🙂