Is your reference desk set up in such a way that patrons have free access behind it? Ours does, and it's a constant, if minor, problem keeping folks from walking through behind us. On the PUBLIB-L discussion list, Joe Schallan has a novel solution.
We set up a Task Force to come up with a solution, and that group quickly determined that we could try a passive solution (barriers and other fortifications) or an active one.
Three active ones immediately identified were poison gas, small arms fire, and mines.
As it turns out, all three have serious drawbacks.
Who would have thought?