This is a very old thread, but since it has been bumped, I will post a final reply.
Slimes only spawn in the bottom 40 levels of the map. That is 0 to 40. They only spawn in certain chunks, which is determined by seed.
This mob spawn regardless of light level.
If you need a plugin, try Essentials. There is a mob disabler function. Otherwise, make the floor of your trap elevation 41.