Probably it should be dealt with structurally, a very small anteroom that only allows access to one person and can't be entered (without obvious force/misbehaviour) until the security door is locked again, like a turnstile before the door. That makes it far more uncomfortable not to challenge someone as they'd have to severely encroach your personal space to gain access.

When we use this technique (known as "tailgating") to break into client sites, we always recommend that the organization try to foster a culture of "trust, but verify". This means employees stopping people if they don't have their badges displayed, or showing unrecognized people to the reception desk, or closing the door behind you to make sure the next person has to badge in, even if they have a badge that looks plausible.

It's not an easy thing to learn to challenge people, but it's vital to maintain a good physical security posture. Employees need to feel comfortable challenging those who they don't recognize, and making sure that employees are part of an organization's security team is important.