Your issue is Partial File Sharing. And you are correct that there is no option to turn this off in LW 5. This is not the same as sharing your incomplete folder. But similar. It's a similar process used for torrents also, that parts of the file you have already downloaded, you upload to others. This overall helps speed up downloads for everyone. And if you are downloading from multiple hosts, chances are some of those are actually partial file sharing the file; the other person does not yet have the entire file. This is one reason why for very large files such as video files, you tend to lose hosts as you get over 90% downloaded. Partial Sharing is fantastic for the network & benefits everyone.

Only LW 4 has an option to disable Partial File Sharing.

There is the other option of Tools -> Options -> Downloads, option of Share files downloaded from the P2P Network with the P2P Network. This means the files get shared after 100% completion of downloading.

I guess in some ways one could argue that the inability to turn off partial file sharing helps stop total 'leeching' by those who do not wish to share any files whatsoever.

(On a side-note, whilst using LW 4.16 I found an odd situation last night. I had partial file sharing turned off, share immediately after downloading turned off, yet I had lots of people in upload window attempting to get a file I was downloading, even after the video download completed. The upload message was "File could not be found". lol Some kind of bug I suspect. ... I share 8,500 file btw. I have partial file sharing & finished downloaded file sharing turned off so I can check the file first before consideration of sharing.)