There are separate sliders for the impact biome has on game stage and loot stage. Think they're on the world and resources pages, respectively.
You can't set them to off but setting them to 'very low' should minimise the effect biomes have on gamestage and lootstage.
Unless you're talking about the lootstage cap per biome? That was removed quite a while ago when the loot system reverted to having per biome modifiers (under the cap system, difficult biomes no longer increased your lootstage, instead they had a higher cap, but the game has gone back to difficult biomes multiplying your base loot stage instead).