You "lock" a table by giving it a set position on a page or in a paragraph.
Right click somewhere in the table and select table properties, then text wrap = around (sorry, translated from a dutch word 2003 version), then "position". Here you can give details where you want to lock the table, and do not forget to deselect the option "move with tekst".
Hope you'll get the point, as I said it is a translation from dutch.