Is there conclusive evidence to refute an earlier oxygenation of the Earth's atmosphere?
The earliest photosynthetic marine organisms (cyanobacteria) are known to have been established by 3.8 billion years ago from the isotopic composition of sedimentary rocks. The earliest microfossils of such organisms are dated at 3.5 billion years. Therefore these photosynthetic micro-organisms were pumping oxygen into the ocean and into the atmosphere at least 1.1 billion years before the Great Oxygenation Event and more likely 1.6 billion. In that almost unimaginably long span of time, these organisms must have created staggering amounts of oxygen. Much of it was soon dissolved and dispersed in the surrounding ocean, but some must have leaked into the CO2 and N2 atmosphere. 

It is impossible to believe that after a thousand million years of oxygen production, not enough had leaked into the atmosphere to give it a measurable amount, notwithstanding the fact that there was no one around at the time to measure it. Some oxygen had been mopped up by iron and other elements from the earliest days, but not enough to show up clearly in the stratigraphic record until the so-called Great Oxygenation Event. It defies common sense to say that there was a sudden explosion of oxygen production 2.4 billion years ago and no significant amount had leaked into the atmosphere well before then. 

So the answer to your question is no, there is not conclusive evidence to refute an earlier oxygenation of Earth's atmosphere. The question is, to what extent did it happen, rather than whether it happened. In other words, what was the percentage of O2 in the earth's atmosphere at, say, 250 million years before the GOE? We may be sure it was far less than in today's atmosphere, but nevertheless a significant amount and (at the time) an easily measurable amount.