Well, I know how to do it in Mozilla (what Netscrape is built on).
Open the download manager, and "Select All". Then hit "Delete" or click "Remove Selected".
As for preventing downloaded files from going into the download manager, all I can say is to get an external manager like:
Flashget -
http://www.flashget.com
Stardownloader -
http://www.stardownloader.com
Getright -
http://www.getright.com