Select the cells, rows, or columns with the line
Go to the Home tab
Click Borders
Choose No Border
If it is a page break line, go to File
Click Options
Select Advanced
Under Display options for this worksheet, uncheck Show page breaks
If it is a gridline, go to the View tab
Uncheck Gridlines
If it is a chart or shape line, select the object
Go to Format
Choose Shape Outline
Select No Outline