Hacking Game Hiscore Tables?

[Gamebrew]

Arg! My hiscore tables keep getting hacked. I was wondering how are people doing it? Also why would they even take the time to do it, but that's a separate issue.

I do send the hiscore data through POST but I also encode it in a string. Also the SWF is obfuscated with SWF Encrypt. So the only way I can think of is that they change the score in memory while the game is playing. Is there such a tool like that, and how can I prevent it? For those who say you can't stop it, I know, I just want to make it more troublesome for them so they think twice. It's a pain to delete hacked scores every week even if it's just one or two.

I already deleted the offending score of 1200, but here's the table:
http://www.gamebrew.com/game-score.php?g=3410

Thanks!