Stopping depends somewhat on what kind of spin you're doing and how you're stopping, but:
1. Think about sending your energy down into the floor to help you slow down and stop. You can use your toes to help this.
2. Since pulling everything in helps you spin faster, the reverse will help you slow down. It's easier to stop with your feet apart rather than together...ie, standing on one foot with the other pointed out to the side.
3. Make sure you're spotting as much as possible...if you're dizzy, it's hard to stop.
4. When you stop, stretch...ie, in rumba: spin, stop on your left foot with your right food pointed. Stretch your left hip back and your right ribcage forward slightly...this will help you to stay balanced and to stop.
5. In something like a spot turn or walk around turn (ie, open break underarm turn where you step across on your left, pivot, replace on your right and end up back where you started), squeezing your thighs will help you to turn quickly and stop quickly.