For starters, don't throw the spin harder than you need to. Often, we're a little nervous about getting around so we spin too fast. From there, the best way to stop your rotation is while you're still in the air. When you 270, fix your eyes uphill at the beginning of the rail, as if you were trying to stop at 180. Let your skis continue to twist to get the rest of the way to 270, but your head will help stop your rotation. Once on the box, if you're still spinning, you'll have to try to pretzel it. For a regular 270, you have to apply some pressure to the toes of your front foot, and the heel of your back leg. Try to get a bit of grip with your edges too. It is actually harder to stop your rotation on boxes than it is on rails because they are so wide...