As Chwala uses kolab_auth plugin we can use kolab_auth_role_settings option for this. E.g.:

$config['kolab_auth_role_settings'] = array(
    'cn=contractor,%dc' => array(
        'fileapi_manticore' => array(
            'value' => null,
        ),
    ), 
);

The above should disable Manticore for users with specified role.

Note that also kolab_files_user_filter may need to be modified to skip "contractors" from searching editing session invitees.