You could use a macro like:
Code:
Sub Demo()
Dim Cmnt As Comment
For Each Cmnt In ActiveDocument.Comments
Cmnt.Author = ""
Next
End Sub
For PC macro installation & usage instructions, see:
http://www.gmayor.com/installing_macro.htm
Do note this will still leave the comment date/time stamp, which can't be removed. The only way around that would be to replace all the comments so they get the current date & time.