When, as a boy, I had to suffer idiots who would clamber over the top of the bathroom stalls when I was in there doing my business, my mom suggested I put ammonia in a squirt gun and shoot them in the face. I declined that (maybe that was fortunate as I learned much later that ammonia could do permanent eye damage), BUT it seems to me a mixture of vinegar and Tabasco sauce would also be effective and do no permanent harm. An empty travel-size shampoo or dish detergent bottle having a small dispenser hole would work almost as well.