Many of the File I/O vi's have a node for Dialog which you can set to False. Make sure you do a Save As... w/ a different vi name for any modification of standard vi's.
For example, using the Write to Spreadsheet.vi, wire false to the Open/Create/Replace.vi Advisory dialog node as shown in the LV 6.1 example attached.
2006 Ultimate LabVIEW G-eek.