The following are off-the-shelf, ready-to-go replies to the vexations of deniers, liars and fools. Sufficiently concise as to fill a coffee break or banter around the water cooler, they could, possibly, have an effect on the recipient.
Climate change denial
First, try reason. Through ample scientific evidence climate change is real and human driven, you patter, with near unanimous consensus among scientists. Science can be trusted, because it has a track record of discovering principles and material facts about the world that make things work, even if they don’t make sense when we use them[2]. Emphasize the consensus being near unanimous is further evidence of the trustworthiness of science, as it would be suspicious precisely if everyone agreed to a fault, such as we find in the cults of religion. Science allows and thrives on descent.
Failing this, argue that the consequences of action on climate change are far less than the consequences of inaction. Even if the human driven component of climate change is only probable, this is still a very wise way to view the future. The prudent principle of spend now to avoid possible catastrophe later informs every insurance policy. We insure our homes against floods and fires, why not our planet, just in case? Though the floods and fires are already upon us.
As a last resort, matter-of-factly point out the benefits to health and well-being of controlling the factors allegedly linked to climate change. Even if the alleged link between human activity and climate is false, the benefits to health of controlling those same factors would save the lives of millions anyway. The clearest example is air pollution from fossil fuels. It does not need to heat the planet to kill us, we just simply need to breathe it.
Hellfire Religion
Favoured by evangelicals in particular, the gates of hell are brandished as an incentive to love and obey God. Belief is not based on reason, but fear. Literally, one is frightened out of hell and into heaven.
In response, simply point out that Satan, too, is said to offer rewards in exchange for souls. That accepting Jesus as your saviour is nothing more than selling your soul to one demon so as to avoid another. And the stunning irony is that Jesus is perhaps more demonic than Lucifer. Lucifer does not threaten the gates of Hell if you reject him, whereas Jesus will hurl you over them into a pit of eternal torment if you do not accept him as your saviour.
Moon Landing Hoax
Casually note that it was far easier to land on the moon than to fake it.
Flat Earthers
There, there. There, there.
[1] Village idiots should not object, as they are frequently themselves shockingly politically incorrect. We need some measure of political incorrectness. The question is how much, and what is the threshold? This is discussed elsewhere.
[2] For example, every time one uses GPS, General Relativity makes it work properly.