The phrase "explicitly warned" is correct and can be used in written English.
You can use it when you want to emphasize that someone has been warned in very clear terms. Example: The teacher explicitly warned the students not to fight on school grounds.
That was a day that saw the worst terrorist attack in American history - an attack that the Bush administration was pretty explicitly warned about in memos and warnings.
Eddington explicitly warned against the use of crude capacity forecasts to lobby for new investment.
He said the president's aides had explicitly warned her to stop her work in Chechnya.
Especially when he has been explicitly warned about how terrible they are.
After two hours, the new friends staggered out of Toast realizing they'd done the exact thing they've repeatedly and explicitly warned their children never to do.
Indeed, the chief prosecutor explicitly warned the government in January that, if it proceeded with its plan, it might invite "sanctions".
But never before have so many bishops so explicitly warned Catholics so close to an election that to vote a certain way was to commit a sin.
And his approval ratings remain sky-high, even though he has explicitly warned that his reforms might produce several years of pain before Japan sees any gain.
Having been poor for a long time, when the money started rolling in he explicitly warned himself: "You could become a real dick".
Johana Mikl-Leitner, the Austrian interior minister, rejected Greek arguments about the difficulties of patrolling its maritime borders with Turkey and explicitly warned Athens about a Schengen expulsion.
He did this even though the exact same report explicitly warned against using these early figures to make long term predictions about the cost of the scheme.
