Your average Invading Alien Species shows up and plans to invade Earth. They are Default Dumb Movie Aliens and don't have nanotech, AI, space habitats, diplomatic abilities, self-replicating factories, slaughter bots, or anything else one expects a Real Interstellar Empire might bring to the table. Their plan is to invade the earth using infantry (because The Author can't be bothered to learn about combined arms tactics), to enslave humanity, and to turn Earth into a colony. This is known to us because The Plot requires us to know it.
You know how this usually goes. The Plucky Hero wins because the Aliens never heard of cyber security. But what if the World Governments decide that there is no hope and Subvert The Expectations of the aliens by committing collective suicide while Trashing The Place? Think of this as a final, species level f-you salute.
How hard is it to do this? My current best idea is to use salted nukes/cobalt bombs, deliberately releasing toxic chemicals and mass producing hydroflourocarbons*. Is this the best strategy? Assuming we commit 30 percent of the world economy to this for a year, how bad would the damage be? The ideal outcome would be toxic, irradiated waste lands under a Venus like atmosphere.
Edit 1: The Aliens want to take Earth intact. They need our environment because The Plot requires them to require it. We know that ruining the Earth in the manner described above will deny them all their objectives. Don't think to deeply about how the aliens work or think, The Author doesn't do that and neither should you.
*I meant sulfurhexafluoride.