Any files under your BitComet directory can be downloaded by other peers, the only way to stop uploading certain files is to remove those files from your directory. However you can change your upload rate via:
Preferences > Connection > Global Max Upload Rate:
Make sure you have BitComet open when you download the torrent. It may take a few secounds (Depending on your internet connection), then you will be prompt with a list of files available under that torrent.
Check out the following topic for more information:
http://forums.bitcomet.com/index.php?showtopic=205
Access Bitcomet > select the Web button > then select the Favourite button. Under favourites is a list of torrent sites, simple access any of these sites, search for your torrent and download.
Go to Options > Preferences. Then select Task from the left colum, you will then see default download path. Here is where all finished and unfinished files are kept, unless they have been moved or deleted.