Microsoft Word Insert Feature

I need to be able to turn of the insert feature of Microsoft Word for certain documents. I’m currently creating computer based documents (boiler plates/templates) of manual documents. These are basically a fill in the blanks document. I need to switch the document from an insert mode to an over write mode automatically. Most of the peopel using the computers to create these documents are not computer savy so I need this as seeming as possible. Sample line included below.

I _________________, being duly

In this example I would want them to be able to type their name where the undeline is, replacing the line, not extending it. Also How cold I make the new text go in as bold underline, or just underline.

Thanks
Bill