The link Charles Kenyon provided contains the answer but here is another way of achieving the same thing. Good luck: -
The ‘line’ gets created after typing 3 dashes then pressing ‘Enter’ but it is not a normal ‘line’. It is a border line. It is annoying that this forum does not allow me to upload a picture from my PC because "a picture explains a thousand words" as the saying goes but, if you click on the symbol that appears on LHS you will see 3 options: -

1. Undo border line
2. Stop automatically creating border lines
3. Control AutoFormat options

If you select option 2 then that should sort it for you.