If the biome is a vanilla biome (I'm guessing you're in Extreme Hills; check the exact biome name by pressing F3) then you can't configure the quantity of the biome. You will have to try another world seed.
If the biome is from ExtraBiomesXL, then press F3 to find the exact biome name. If you'd like to keep the rest of the world, also type in /seed and write down the seed so you can later use it to re-generate the world (it may cause glitches if the game asks for a biome that can't be generated when you re-open your world after modifying the configuration file.) Once you've done that, navigate to the ExtraBiomesXL configuration file (.feedthebeast/MindCrack/minecraft/config/extrabiomes/extrabiomes.cfg) and open it in a plain text editor (I prefer Notepad++, but Notepad and possibly Wordpad will also work) and edit the following line:
Replace "true" with "false" when you're editing the file, and replace "BIOMENAMEHERE" with the name of the biome that is giving you problems.
Unfortunately, there is no way to configure the exact chance of a biome spawning, as the chances of a given biome spawning differ from seed to seed. This is something where you can either have the biome or not have it.