Hi Dave,
The reason why you get these boxes is the next.
If you edit a text and press delete, you assume that the text is completely deleted, but no not with autocad. To completely delete text items you need to use the erase command (there needs to be text in the item).
We had the same problem in our company, but we where looking for a reason why some of our drawings took so much disk space. What happened, when you have blocks, or drawings with such items in it and you insert them into other drawings well .......... By using the Qtext we managed on some drawings to reduce the size by 50%.
Sidenote: To Autodesk. Maybe in one of you next releases you can take care of this problem.