it:ad:qgis:howto:hide_things_in_print_output_legend
Summary
The legend works poorly. Can't hide contours for example.
The alt way is to put another map on the page, maybe very small, associated to one of a set of map Themes that you have named 'Legend-XYZ' with just the main features you want in the legend (e.g. overlays, etc).
Make the Legend's Properties/Display/Do not Export.
Check the Legend's Properties/Frame box, and give it Red or other Bold colour.
Print and check.