What I am trying to achieve with this is that certain races are only able to colonize certain planets. A planet doesn't have to have the IsExtreme parameter as prerequisites for colonizing is a different parameter.
In the GalCiv3MapDefs.xml file there is the 'PlanetTraitLikelihood' parameter, which is currently set to .75(% I guess). Thought that changing this to 1 would do the trick. Nope. Even 100 and 10000 changes nothing or little. However setting it to 0 does indeed remove the possibility of a planet with a trait appearing. I then tried all the extreme planets parameters, which acted in a similar matter.