You're just angry at god
"You're just angry at god" is something religious people accuse atheists of in order to dismiss their lack of belief. The obvious problem with this accusation is, you can't be angry at something you don't believe; atheists aren't mad at gods for the same reason they're not mad at leprechauns. However, I don't suggest arguing along these lines. Instead, it's best to first understand why religious people make this claim to begin with.
The reason you hear this dismissal is because religious people frequently doubt the existence of their gods after a traumatic event happens in their life. This is because no religion has come up with a comforting answer to the question, why do bad things happen to good people? However, religions attempt to inoculate against such a crisis of faith with stories of their god allowing his faithful followers to be tested in traumatic ways, and, when the tormented people maintain their faith, they're rewarded greatly for their steadfastness.
For example, in the Book of Job, Elohim boasts about his faithful servant Job, but Satan argues that Job is only faithful because Elohim has given him so many riches. So, Elohim allows Satan to take away Job's wealth, torture him severely, and even murder his children. Job's wife tells him to renounce his faith and die rather than remain faithful and be tortured, and, it's assumed that Job must be awfully angry at Elohim for allowing all these terrible things to happen to him. However, Job remains faithful, and, in the end, Elohim rewards Job's faith with new children and even more worldly possessions than he had before. A modern example is the Evangelical film God's Not Dead where an atheist professor who tries to convince all his students that the Christian god doesn't exist eventually lets it slip that he actually does believe in the Christian god and is angry at him because his mother died even though he prayed for her to be saved.
Religious people view apostasy through such a lens. To them, when a former religious person denounces their faith, they frequently assume something terrible has happened to the person, but, unlike Job who remained faithful through the trial, the fallen believer decided to denounce their faith to punish the Christian god, even though they still believe. It is for this reason I don't think atheists should respond to the accusation by saying "you can't be mad at something you don't believe;" the religious people who use this accusation think the atheist secretly still believes their god. This is also why religious people don't usually say this phrase to someone who deconverted from a fundamentally different religion; you won't hear a Christian tell a former Hindu that they're just angry at god.
I think a better approach is to ask the religious person why they think I'm angry at their god, and, if they explain how people lose faith due to traumatic events, I can then explain to them that nothing that bad has ever happened in my life, instead, my atheism was brought about by learning more about various subjects like history, mythology, biology, physics, cosmology, and philosophy. I've spoken to many other atheists about why they deconverted, and I find that this is by-and-large the most common reason. It's not about a trauma, it's about education. When you confront their accusation in this manner, they can't so easily fall back on their preconceived belief of you being angry at their god, and must reassess your atheism.