This emphasis is underscored in multiple training classes. For example, every new analyst must attend the Career Analyst Program (CAP), where grizzled intelligence vets teach "the basic thinking, writing, and briefing skills needed for a successful career." One point that gets hammered home is what happens to people who provide information to those who shouldn't have it -- especially foreign governments. These classes highlight, among other cases, the Aldrich Ames and Robert Hanssen affairs, and take care to emphasize that these former top officials are currently serving life sentences in prison.
It's the Agency Culture. CIA employees are reminded in ways both large and small about the consequences of mishandling or misusing classified documents. Your colleagues remind you. Your managers remind you. The internal websites remind you. When someone is caught providing secrets, even the director reminds you.
Furthermore, because of their chosen careers, CIA employees are made justifiably paranoid about "security violations" -- for instance, if you absent-mindedly took a classified document from your office, placed it in your briefcase or purse, exited the building, and then remembered you had it while walking to your car, the Office of Security could slap you with a security violation. (Pro tip: Don't take a suitcase or large purse to work.)
There are many other ways to trip up. If you didn't correctly seal a secret document while en route to a meeting, it would be a violation. Or casually mention a friend's real name publicly who might or might not be undercover. Any of these violations could cause you to lose your security clearance and your job.
Certainly, if a person is considering providing classified documents to the press or a foreign government, he or she is probably far, far down the path to breaking the law. Edward Snowden had been considering leaking top-secret documents for years prior to actually doing it. It is doubtful that any seventh floor-directed educational effort would have steered him away from a lonely trip down the road to treachery.
CIA's classified disclosures are usually to foreign governments. Compared to the military and other civilian agencies, CIA has a decent track record of keeping secrets. Of course, moles in the agency have done grievous damage to American security, like Aldrich Ames and Harold Nicholson, who both spied for the Soviet/Russian intelligence services, and translator Larry Wu Tai Chin, who secretly worked for the Chinese government for decades. Even Ghana's intelligence service during the 1980s penetrated CIA.
Granted, there are a few CIA officials who go to the press, but their subsequent punishments have varied. John Kiriakou is now serving 30 months in a federal prison after providing the name of undercover officer. But senior CIA official Mary McCarthy, who leaked the existence of "black sites" housing captured al-Qaeda operatives to the Washington Post's Dana Priest was fired, but she was planning on retiring anyway -- and served no jail time. And of course, Edward Snowden was once a former CIA computer technician.