Vandals might think they are clever (they aren’t) but 3M has them beat with the 3M™ Anti-Graffiti Film.
We commonly use the Safety S40 anti-graffiti film to protect against purposeful and accidental glass damage. It will either stop scratches and other marks or if damaged it can be replaced with a new film instead of your windows. It has a strong acrylic adhesive which fixes it securely on to whatever glass you want to protect and will also cover other smooth and flat surfaces like lift interiors or polished granite walls.
The high strength polyester film resists general wear and tear as a matter of course and is weatherproof. The high UV protection also prevents fading in shop windows and is virtually invisible. The vandals won’t even know why they can’t mark it.