That's a good question, and I don't think it's possible to lock tables (I've never really thought about it)?
You could do what we used to have at uni and make 'forms', where the user can only type in a section that is clicked on. I remember seeing some tutorials about on google, but it might not be what you're looking for.
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.