There is not currently an option to disable the Hanar and Drell separately. However, if you are somewhat savvy with xml editing, there are separate factions in the files for the Hanar and Drell, but they are "non-spawning". It shouldn't be too difficult to disable the Enkindler faction (in the files, not the settings since that would disable the pawns as well) and re-enable the separate Drell faction. I haven't looked at them in a while though, so you may need to update some stuff to make it work. I only use the combined faction because of the limitations of Hanar colonists.