The category legend can be hidden using a css override.  In this case there is one already provided.  Add the following to end of the map url:

?css=https://www.easymapmaker.com/cssoverride/hidegroup.css

The following map url:

https://www.easymapmaker.com/map/my-first-map

would become:

https://www.easymapmaker.com/map/my-first-map?css=https://www.easymapmaker.com/cssoverride/hidegroup.css

Css overrides allow for the css on the map to be changed to allow further customization to a map.