Glass would be much more expensive than paint and would require cleaning more often plus glass is cold and soulless. A glass mosaic maybe and the paint could be cleaned from it but it would completely change the vibe and it's not worth doing something so very drastic for a short term problem. Education is the right way to address the problem. One issue is that the color of the wall's paint changes a bit in time and you can't paint only over the graffiti with the original color but that could be addressed with an automated tool that scans the color and mixes the right tone of paint - tool that likely does not yet exit. Anyway, it's good that you are looking for solutions to problems.