Minecraft wiki entry for Managing_slimes_in_superflat_mode, associated text: This simple trick can be achieved by using a repeating command block with the setting always active on. In the command block, use this simple command: /kill @e[type=minecraft:slime] or /tp @e[type=minecraft:slime] ~ -70 ~ (The second command stops slimeball drops by teleporting the slimes into the void)