The advantage of BitTorrent over other file distribution tools, say rsync, is that clients start to offer their data to the other clients, so if the server has a slow connection to the clients, throughput will be much better.
BT is a protocol which is designed for global sharing, though. So you will have to find a means to lock out all other clients but the 7 you want to feed (or your torrent will soon end up on The Pirate Bay).
As for stopping at 100%, every BT client will do that: Since no client requests the data anymore, there will be no more seeding. The application will still run but they will only cause a little trickle of data between the clients and the tracker to tell it that they are still available.
Try the original BitTorrent client which is written in Python. This will allow you to make any modifications you desire. Unfortunately, I can't find a link ATM. Anyone?
[EDIT] Based on a comment saua: If you use a private tracker, your torrents should not spread unexpectedly. That said, you should only use a torrent client from a reliable source. Otherwise, you might risk that some cracker has put a little extra feature in the code which publishes private torrents somewhere where you don't want them to appear.