For me it's not that I have incontrovertible proof to demonstrate that no god(s) exist, it's just that in most cases I've come across the "proof" that god does exist is only really proof of anything if you're not imaginative enough and dedicated enough to come up with alternative (and in most cases far simpler and more satisfying) explanations for things that are typically explained as being due to god.
In some cases there are things we still don't fully understand, but looking at the past progression of science every time someone hits a problem and throws up their hands and says "well it must be god that's making this work!" we later find and understand the mechanism behind it. Given the trend, it seems likely that as our knowledge of the universe progresses there will be nothing left that someone can claim god was responsible for. Or at the very least, the things that people will claim god is responsible for will have to be wholly different from what they currently claim he's done.
Given the trends, I feel that I can say with a high degree of certainty that there is no supreme being out there, and if there is they certainly aren't deserving of my worship based on a few ancient religious texts. The god of the bible least of all.