Try crimping the inside to outside of each side of the slider a little with a pair of pliers. Just a little though... if it's a cheap zipper you could easily break the slider in half.
Jeans zippers often have a little brass (or whatever ****l) projection that sticks into the zipper teeth when the pull is down; that might have worn off. But the more dress types slacks I'm wearing at the moment don't seem to.
Another thing you might try is to get some of the "carpet/button" thread and put two little hand stiches just outside the teeth just below where the slider would be when it's up, with the idea that these might be require willfull intent to get the slider past. You could also try raising a burr on the outside of teeth a little just below where you want it to rest, with pliers if they're brass, or with something hot if they're plastic, but that's probably a bit more risky.