Select the line you want to delete
Press Backspace or Delete
If it is a horizontal line, click directly above or below it and press Backspace
If it is a paragraph border line, place the cursor in the paragraph and remove the border from the Borders menu
If it is a table line, select the table row or border and delete it from Table Tools
If it is a drawn line, click the line and press Delete
If it is a blank line, place the cursor on it and press Backspace or Delete