My parenthetical is important. The "it doesn't [want to]" is precisely why many ideas won't work, because many adversaries within the species (the rest of us) will work against anyone trying, and getting enough people with the know-how and desire to try will be hard. In fact the hardest part of the fragment you pulled out is developing a strategy that will indeed end almost all life without first killing all humans. A lot of strategies are likely relatively long-term (and inplausible to actively work on because of adversaries, however some believe human activity is already an ongoing inadvertent mass-extinction event for other life and maybe ourselves eventually). However the thing in my mind when I wrote that was intentional asteroid impact. We've already demonstrated we can land on the things, NASA's been busy finalizing the details of a redirection mission, so redirecting a sufficient number of smaller ones or just one really big one (but why stop at one?) ought to do the trick for most plant and animal life.