Here's how to remove the little dotted lines that sometimes appear on the spreadsheet.
Specifically, this deals with removing or hiding the print preview lines that appear in Excel after you have adjusted a setting for printing but then gone back to the normal view of the spreadsheet.
Here, you can see the annoying print preview dotted lines between columns I and J:
This only works on the currently visible worksheet. If you want to hide the print preview lines on other worksheets, you will have to do that separately for each one.