Shrinking the exhaust? Covering up the airbox? Changing the gearing? Using a washer on the Carb?
What I'm hearing/reading is, it sounds like, a 500cc two-stroke is not for you. If you like bigbore power at less-a-hit, try the XR series bikes. If you are just a two-stroke guy, try a 250. But, I have to say, my KX250 hits harder than my 500 (well it is felt more, due to traction). I'd just suggest looking for another bike. You can't reduce ""POWER"" persay, but you can change the power. Anything you do to reduce power will damage the engine in some way. The BIG BORE has to breath, you can't restrict the air without hurting it. It will lead to issues with your block. Read the issues already about vaccum to the KX and the surging.
I'll leave this post alone, but I will say, don't buy anything you can't handle. If you have to reduce the power on a bike you own and can't just control yourself or the throttle, get rid of it and start smaller, then once you feel more comfortable on what you chose, move up to the next size.