Plenty of good suggestions here, but can I add one that might mess with their minds and have them consider going elsewhere but is still easy to overcome. It's something that was done in my earlier apprentice days, to us or by us, to peoples tool boxes. You need to have a suitable steel structure next to something that is attached to the van permanently and can take a large bolt. Take one large diameter bolt and cut off about 20mm of thread and place into nut by a couple of threads and place one or two spot welds to lock them together. Pass bolt through both brackets and place welded nut, with some Loctite, on to end of bolt and tighten as hard as you can. Haven't seen it fail for the victim to spend ample time trying to cut off bolt with hacksaw only to be shown later the easy way. This was done in days prior to fun being outlawed in the workplace. Of course I never did this.