Disabling or Freezing links in Excel 2K

Is there a way to disable or “freeze” linked cells between Excel workbooks and sheets? We are using Excel 2000 and have series of Excel workbooks and sheets that have cells linked to cells in another Workbook/sheet which is used for rolling up financial information from operating companies to the parent holding company. At certain points in the roll up porcess we would like to stop charges made to operating company workbook from changing the linked cells in the parent company workbook. Is there a way to accomplish this?