AFAIK you cannot uninstall or disable Forbidden Worlds.
I made a mod that imitates disabling Forbidden Worlds:
https://forums.sinsofasolarempire.com/429281
If you have some mods that somehow are not meant to work with the DLC (eg., a mod with new planets that do not allow specializations), maybe you could try putting the Trivial NoDLC mod at the BOTTOM of the enable list, such that the NoDLC mod would attempt to un-DLC things that are not already covered by your mods. I have no idea how well this would work, and YMMV!
