If a hostile group wanted to render Earth or an Earth-like planet completely unfit for human life, what could they do to go about achieving that end?
By uninhabitable, I mean a planet on which humans are unable to survive in the open for the foreseeable future. Centuries in the future, whatever's happening could calm down and maybe allow for habitation again, but after the event, nobody could survive. The group who is rendering the planet uninhabitable wants to salt the earth, so to say.
The two ways I've thought of that this could be done would be to create a Venus-like runaway greenhouse effect, or else to strip away the atmosphere, but I'm not sure how people could actually do either of those things. Other methods that I considered were some sort of biological agent that would be lethal to people or a nuclear war. These both seem like they'd be able to kill most people on earth, but radiation dies down fairly quickly and biological agents can have cures.
I'd like to limit things to solutions that are feasible using modern science, and would also like to avoid the outright destruction of the planet. (So no crashing the moon into the surface.)
What convincing ways could I have such an attack on Earth render it unfit for human habitation?