I was able to recreate this issue by creating a report using the wizard. To get rid of the line, you need to select all, (control A), then with the arrange tab on the ribbon selected, click on the remove icon. This separates all control movements from being in sync and eliminates the lines in question that separates the body of the report from the header.
I suspect that this was not obvious to most experienced forum participants because they seldom use wizards to create reports, forms or queries.
Alan