An easy to install (but hard to remove) desk lock. Open the drawer, and use two screws to install the lock near the front, just below the top lip. Once installed, slide closed.

The lock will be flexible enough to deform and let the drawer close. However, the catches on the bottom will prevent this from happening when someone tries to open the drawer. The low profile will make it easy to mistake the jam for some stuck office supplies.

Install, bide your time, and observe the chaos.