Sometimes , Kalevi, it has to do with how you created the line in the first
place:
One shortcut method for creating a line WHICH LINKS IT TO TEXT AND THEREFORE
MAKES IT IMPOSSIBLE TO DELETE ON ITS OWN is to type three hyphens (---) and
then press enter. It is also possible to create other versions of a line
this way, including three equals signs (===), press enter; three number/pound
signs (###), three asterisks (***), or three tilde (~~~). If this is how
you have created your line, then yes, you'll probably have to delete the text
linked to it (or the whole file) to get rid of it.
Try using Format | Borders & Shading | Borders on selected text if you
want a line. It will give you a wide variety of options forline styles, all
in the format of a four-sided border. Click on the edge(s) of the box in the
Preview window on this menu to delete the top/sides/bottom you do not want.