In the biomes.xml.For each biome you'll need to locate

example)
<decoration type="prefab" name="
rock_form02" prob=
"0.0007" rotatemax="4"/>
<decoration type="prefab" name="
rock_form01" prob="
0.0007" rotatemax="4"/>
I can't recall which rock form that specific one is though but adjust their prob="" down.
you'll need to reroll a new world for the change to take affect though.
The example is what I use in some biomes..