We might not be able to conclusively prove that there is no entity which might reasonably be labeled a god, but we can disprove any claimed evidence for his existence. For instance, miracles. Many people claim to have observed miracles, but this can be seen to be simply the tendency of people who desire the extraordinary to see it at work when there are only ordinary forces at work. The same type of phenomena has often been observed in claims of psychic power. A self-proclaimed psychic will make predictions and make claims about reality that often fool people who are inclined to believe in psychics that they do, in fact, have psychic powers. Famed skeptic Michael Shermer (publisher of Skeptic magazine) once did just that - for a PBS science series, he pretended to be a psychic, and performed cold readings on several guests, who thought they were simply getting the chance to be interviewed by a psychic for a television show on psychics. He used a variety of props - star charts, tarot cards, palm readings - or none at all, and convinced every single guest that he had psychic powers. The miraculous is no different - it takes little to convince someone predisposed to see miracles that what they saw was miraculous, and a great deal more to convince a skeptic. There have been no known cases of skeptics convinced that they saw miracles, and yet thousands of claimed miracles observed by the faithful. Occam's razor says that miracles simply don't exist. The same thing goes for the power of prayer. Prayer is often claimed to aid in healing, but medical studies have been done, and no evidence for this claim has ever been found.
In other words, there is no evidence at all for the existence of a god - all of the arguments for his existence based on pure reason can be seen to be false, and all of the arguments based on supposed evidence are likewise observed to be false. This puts god in a category with the flying spaghetti monster, the invisible pink unicorn, and the celestial teapot in orbit of saturn. You don't require any evidence to dismiss the existence of these objects, so why do you require any to dismiss the existence of god?