Config Options

Currently, Forge does not have the config ui implemented yet. So unfortunately if you want to change something, you'll have to dig through a very confusing config file.

For biomes:
Biomes are picked based on weighted values, meaning higher weights give the biome a higher chance to be placed. The config has weight options for every biome and will allow a range of 0 to 100. For reference - 10 is the default for most common vanilla biomes (forest, taiga, etc.). I always have mod biome weights less than that because I prefer to not have all these new biomes overtake what should be a vanilla-esque world.
Setting the weight to 0 will disable the biome.

For world gen/mob spawns:
Everything related to world generation or mob spawning will have an option that is a list of biomes where they will be found in. In the case of some (not all) trees, adding a biome to this list will cause the tree to generate there. To disable a mob or some other feature, remove all biomes from its spawn list. Biomes are always in the format of ModID:BiomeName. This also means you can add other mods' biomes. You can find a list of vanilla biome IDs in the Minecraft Wiki