I have no problems with others jumping on my torrents. It simply means that others can download faster. Unless it is a very low demand torrent, I can continue to seed for the x number of hours I had planed in the first place and upload the same amount as if no one had jumped on. So I really do not lose anything. After all it is the amount uploaded that counts and not whether it was initial seeding or a very old torrent.
I have not been a member for long, but in my experience the seed bonus given to me as the torrent uploader has probably been more useful then the actual amount I have uploaded.
Consider the new user who has submitted a torrent and after waiting 10, 12 hours or more sees you come along with auto-approval and upload the exact same torrent he has waiting. He put just as much time and effort into getting the torrent ready as you did. Don't you think it is fair that he should be able to upload as well?
As there is no easy technical solution to control the jump-on, why not just embrace it. :)