I've concucted a plan to sabatoge the leechers. What you and your party agrees to do is simple and only requires a small sacrifice.

1) Join the caravan as usual.
2) Let the leech run to the end.
3) Hold the caravan at the very beginning, defending it if need be.
4) 5 minutes or so from the end, everyone abandon the caravan and run back to the Caravan administerrerr.

What this does is cause the leecher to fail without knowing that anyone dropped the caravan. He would have to run back or return back. The time would run out to 15 minutes, leaving the leecher but a few seconds to catch the next one. By then, you and your party would have already signed up and filled the spots.
Justice is then served.