See, Ohio State has a policy against such things happening:
Firearms and Weapons Policy
No person shall be permitted to carry or possess a concealed or unconcealed firearm, weapon, or other hazardous materials on the university campus, other premises owned or within the control of the university, or at functions sponsored or sanctioned by the university. Items covered by this policy include, but are not limited to the following:
Guns and firearms of all types
Sure is a good thing they had that policy in place! Otherwise someone might get hurt!
But anyway, if someone actually decided to [gasp] violate this policy and bring a gun on campus, Ohio State's "Dealing with Disruptive and Distressed Individuals" policy should handle things just fine:
How should I deal with a disruptive person?
Disruptive behavior should not be ignored. Remain calm. Remind yourself that it is not about you, it is about the situation. Tell the individual that such behavior is inappropriate. Inform the individual that there are consequences for failing to improve the disruptive behavior. Many disruptive situations involve anger. Recognize that the period of peak anger usually lasts 20-30 seconds.
Although this may feel like an eternity in the throes of the situation, often it is best to “wait it out” before progressing. Remember to keep your supervisor or department chair apprised.