A favorite word for Campus Security's level of strictness is "lax," which pretty much speaks for itself. Campus Security officers are more concerned with safety than with being policemen. They aren't out to ruin people's fun. There are always rumors of certain officers partying with students, but most of them play it pretty safe. One thing Pomona tries to be harsh on is vandalism, which has become an increasing problem in the last few years. So while a Campus Security officer might very well turn a blind eye to your not-so-incognito brown-bagged forty, he or she is not likely to turn the same blind eye to your attempts to break a dorm window with your head.
The most common cause for parties getting shut down at Pomona is a noise complaint. Generally, if there is a lot of noise being made anytime after quiet hours, some spoilsport (or maybe a perfectly nice person who just happens to have two or three finals to study for) will complain. The RAs (resident advisers) will respond first. They are students who are granted a certain amount of disciplinary authority by the College, but mostly they just let people into their rooms when people forget their keys and go to parties and tell people to be quiet or else "I'm going to call Campus Safety!" There is a pretty even mix between laid-back and uptight RAs, and if you end up with a few of the former in your dorm, you're home free (unless of course you're one of those aforementioned people who place a high priority on sleeping and studying). Pomona's administration is generally very fair, as well, and prefers everything to be handled on campus as opposed to by the police. You won't get hassled as long as you apply a modicum of common sense and courtesy.