How can we get rid of slimes on super flat?

/kill @e[type=Slime] will kill all the Slimes in the world while leaving everything else alone. With this command in a Command Block, an automatic elimination of Slimes is possible: Just set it to Repeat and Always Active, and then you're done.

You can change your difficulty to peaceful.

You can create a redstone loop + command block system to constantly remove any unwanted entities(Mobs, Items, Item frames, etc.)

First, create your RS loop (shown below)

Top-down view [r][#][B] [r][r][t]

Key:
[r] = Redstone wire (dust)
[B] = Any solid block, wool works fine
[t] = Redstone torch
[#] = A redstone repeater set to 3 ticks

Now connect any number of Command Blocks (to get Command Blocks enter /give _USERNAME_ minecraft:command_block 1 into the chat) equal to the amount of entities you want to be cleaned up to the redstone wire on that loop.

Enter the following command into the Command Block(s) connected to your loop, replacing _ENTITY_ with the name of the entity you want removed (ei. Cow, Bat, Pig, Spider, Skeleton, Slime):

/kill @e[type=_ENTITY_]