From the Microsoft knowledge base:
Markup text may reappear when you open or when you save a document of Word 2007
When you open or save a Microsoft Word document, markup text (comments, ink annotations, insertions and deletions, and formatting changes) that you turned off in the document the last time that you saved it may appear again.
To change this behavior and to retain the Reviewing options as you did in Word 2002, follow these steps, as appropriate for the version of Word that you are running.
Microsoft Office Word 2007
Start Word 2007, and then open the Word 2007 document.
Click the Microsoft Office Button, and then click Word Options.
In the left pane, click Trust Center, and then click Trust Center Settings.
In the left pane, click Privacy Options, and then click to clear the Make hidden markup visible when opening or saving check box.
Click OK two times.
Make a change to the document. For example, type a space, and then delete the space. Then, save the document.
Note If you do not perform step 6 and make a change to the document, the change that you made to the Make hidden markup visible when opening or saving option may not take effect the next time that you open the document.
Note If you save the file with the Make hidden markup visible when opening or saving option cleared, another user will see the markup text immediately when he or she opens the document. The other user must also clear the Make hidden markup visible when opening or saving option.
This is because when you open a document that contains tracked changes, the document appears in the Final Showing Markup view to indicate that the document contains tracked changes. This behavior is by default.