In general we're pretty relaxed at my library, and try to err on the side of good customer service. However, as the empty cases built up, staff started investigating ways to curtail the thefts.
But the kicker was that, when we ran the numbers, all of the security options we looked at (cameras, dummy cameras, security cases, a DVD jukebox, keeping DVDs behind the desk, etc.) were actually more expensive than just buying replacement DVDs. At least, this was true for the rate of theft we were seeing.
It seems counter-intuitive, and a little aggravating, but this is the route we took. The Circ staff was especially frustrated by the apparent "do nothing" approach, but we reviewed the numbers multiple times over the years, and replacement was always the cheapest option. Well, that combined stepped-up monitoring by staff.
And then something happened that no one expected: a stack of DVDs with a note attached ended up in our bookbox. Apparently, whoever had been stealing them got a conscience (or else, as one popular theory holds, his mother found them*). And then, a week later, a second stack of disks showed up.
We had been saving the empty cases all along, so re-adding them to the collection was easy. Hopefully, this trend will continue, and we'll end up with all of our DVDs back - just a couple years late. And we haven't noticed many missing lately, so the increased staff monitoring also seems to be working.
*Most of the DVDs that were stolen were Adam Sandler/Will Ferrell/American Pie-type movies, which implies the culprit(s) is probably high school boys.