Go to the View tab
Uncheck Page Break Preview if it is selected
Go to File
Select Options
Click Advanced
Under Display options for this worksheet
Uncheck Show page breaks
Click OK
Select the cells with dotted borders
Go to the Home tab
Click Borders
Choose No Border
If the dotted lines are print area borders
Go to Page Layout
Click Print Area
Select Clear Print Area
If the dotted lines are from copied cells
Press Esc
Click a different cell
If the dotted lines are from selection outlines
Save the file and reopen it
Restart Excel if needed