There is no option to disable password field. But this code may work. Please try it:

function my_custom_account_fields( $fields ){
    $remove_fields = array('user_pass');
    if( !empty($fields) ){
        foreach( $fields as $r => $rows ){
            foreach( $rows as $c => $cols ){
                foreach( $cols as $f => $field ){
                    if( wpfval($field, 'name') && in_array($field['name'], $remove_fields) ){
                        unset($fields[$r][$c][$f]);
                    }
                }
            }
        }
    }
    return $fields;
}
add_filter( 'wpforo_get_account_fields', 'my_custom_account_fields');

Put this code in current active WordPress theme functions.php file.

How to Easily Add Custom Code in WordPress (without Breaking Your Site)

It might be easier (especially if you want to revert at some point) to "pointer-events: none;" the password fields in Style Settings CSS?