You can't Stop them. You can only Discourage them. So please reformulate your question.
And let me reformulate mine so you couldn't avoid answering it for a 4th time: how were the perpetrators caught?
Sigh... Ok you are taking one portion of one incident and breaking this down into pieces so you can argue points instead of the policy... you have a good future in politics, then again we all do. (no that was not meant as a personal attack, I apologize if it is taken that way)
Campus Police are trained to observe different activities of individuals. Not only those that are actively ingesting a particular illegal material, but those that are even carrying said product or even those that are looking to purchase said product act differently than those that are "just going about their business". I for one have taken classes (Criminal Justice) in spotting suspicious activities and it is very easy on about 80% of the time.
As a very easy example if you are a parent and you walk past your child you can usually tell that they are/have been/about to do something wrong. They just don't act right. This in itself is not just cause to arrest or detain said individual but it does allow them to question that person.
In the college that I am aware of there were multiple individuals sent to the hospital due to injuries as well as adverse reactions to substances that were added to the juice. There were even more instances where the Campus Police questioned the individuals and they admitted to the substance.
Your campus may be different, but if it was one campus it would be different, but this is gaining wind due to more issues like those above.
Me personally, I would love to be able to vape anywhere at any time, and push to do so. But I can understand why College campuses decide to do this. As one professor said "Not all cars driving on the sidewalk kill people, it only takes one to make it bad for everyone else"