I am running a small on-line contest too... and I just decided from the beginning that you can't take it too seriously because it is so possible for people to cheat.
So mine is aimed at people who just want to share their stuff, and who don't enter too many contests.
I thing if you try to run a real, big, official type on-line contest (like an IPMS) you are never going to get past the problems of people "cheating".
Just make it a FUN thing, and don't worry about what people do, cuz you will never be able to monitor your entrants unless you go to everyone's home and check for yourself!!
www.naplak.com/modeling ... a free site for modelers
www.scalehobby.com/forum/index.php ... a nice Modeling Forum