Short of the LAPD putting out a city-wide tactical alert (think Rodney King riots), you're never going to put a serious dent in these situations. They saw cops, they moved somewhere they DIDN'T see cops.
Then again, CA is a "may-issue" state, which is to say they don't give ANYONE a CCW permit for shit. The OC Sheriff wants to pull about 80 percent of the CCW permits, claiming that Sheriff Carona just gave them out as "favors". Problem is, that'll leave about 300 people in Orange County with legal CCW permits.
In a county of over 3 MILLION. LA County has a population of over 10 million, and Sheriff Baca has issued 400. That's right. 400 people in a county of 10 million, or 0.004 percent.
Which all leads me to the words of Heinlein: "An armed society is a polite society. Manners are good when one may have to back up his acts with his life."