100% positive there is no God?
Well, no - but, I'm not 100% positive about anything.
I'm fairly certain that all the "deity" and "creation" myths are just really myths - as most of them are simply absurd and have no supporting evidence.
As to "proof" - ultimately, one can only "prove" mathematics and gain evidence for the existence of something. You can't "prove" something doesn't exist, you can only gain evidence for it's contrary.
I agree that they are agnostic rather than atheist, but I think "coward" is an unnecessary and inappropriate smear in most cases. I don't think most Ivory Soap Atheists (99 44/100 pure) hold back out of fear, but out of a contrived notion of fairness.
Many were reared as theists and recall how certain they were that there was a God and yet appear to have been mistaken. That provides an understandable grain of salt and they fail to discern the difference between being programmed as a child to accept something on faith and falsely but sincerely calling it knowledge, and using logic, that is a system of thought based on non-contradictory data, and arriving at conclusions based on such.
They are further held back by such homespun bits like "you can't prove a negative," which many take as a rule of logic even though it is clearly a fallacy.
Then they tie themselves with the notion that certainty is a magic spell that will prevent them from having later epiphanies should contrary evidence present itself.
I suppose that they imagine some scenario where those of us who are certain are wandering through the rapture with our eyes closed unable to see the risen Christ as he gathers his elect or some such. They pretend that being certain is a vice and never a virtue. They are told that they must be open-minded in order to be scientific, that they must allow the possibility that any random assertion by any nut on the street might be true. Of course, science does not make such a demand anymore than medicine demands that doctors follow the Hippocratic Oath in order to heal. It is a contrivance sold to those who do not think for themselves.
It does not logically follow that being approachable by those claiming new evidence for old assertions demands that one never have been certain.