It’s simple: With armed force.
[Jeanne] Assam, a church security guard with law enforcement experience, fired her own weapon at the invader and stopped his attack, police say. . . .
Assam was one of about a dozen volunteer security guards at the church, half of whom are armed, [Pastor] Boyd said. The guards are licensed, trained and screened, and are church members, not “mercenaries,” he said.
What would have happened at Virginia Tech last spring, if the school had allowed some of its faculty, staff, and even students to be licensed, trained, screened, and armed when Cho went on his rampage?
UPDATE: Here is a detailed eye-witness account of how the whole thing went down.