The phrase "decisively proven" is correct and usable in written English.
You can use it to describe something that has been firmly established or conclusively shown to be true. For example, "The connection between diet and heart health has been decisively proven."
But you can't decisively prove how the economy would have behaved without the stimulus.
Improbably, that steadfastness can also still be thrilling, as Tuesday's concert decisively proved.
The Platonic vision can persist in economics because economic models are mathematical (in that sense Platonic), and you can never decisively prove them wrong.
But he decisively proved that supposition wrong with "Written on Skin," which was enthusiastically received at its premiere in July at the Aix-en-Provence Festival.
Ironically, in areas of science where researchers believe that a new piece of evidence might decisively prove them wrong, they are perhaps more circumspect with their suggestions.
How Important Are Eye Movements in E.M.D.R.? Have you been able to decisively prove that the eye movements themselves serve a true clinical role, and if so what evidence have you garnered to show this is the case?
My early version of Pirandello's The Rules of the Game, written when I was 23, and performed by Paul Scofield and Joan Plowright for the National Theatre, decisively proved that I knew nothing about Pirandello and still less about adaptation: it was inept.
By way of decisively proving that procrastination is the enemy of rock, there is one clincher: the Kingsmen's epochal 1963 single Louie Louie, aka the Greatest Record of All Time, audibly recorded in one afternoon, and all the better for it.
(We know, of course, through Scripture that the world is not eternal, but we could never decisively prove this by means of unaided human reason).
Observations made years earlier revealed hints of the planet but could not decisively prove its existence.
