Quote from Deacon_Blue
to upload a screenshot, go to imgur or something, choose "upload a file" and then search for your screenshot.
As for the real question at hand - Slimes spawn in ~1/10 chunks which are chosen by your world seed. In superflat, the only REAL good ways to prevent spawning are half slabs. You can use double half slabs that act like regular blocks but still prevent spawning.
On grass, you can use bonemeal to force tall grass to grow or use torches in literally every square. They can't spawn on blocks that have stuff in them (torches, grass, redstone, etc)
Double stacked half slabs still allow spawning for any mob. I made a spawner that abused that in 1.7 in order to farm endermen (before they flipped out over water, and could still move any block).
OP: This page has a list of all blocks that will prevent mobs from spawning: http://www.minecraftwiki.net/wiki/Opaque#List_of_things_affected_by_Opacity
Keep in mind that you need one of these blocks on EVERY square in order to prevent spawning. Tiny slimes can still spawn in 1x1 areas, while medium and large slimes require 3x3 areas.