Simple and cheap anti-graffiti layer based non-paint solvent
The "professional" solutions here in the US are often fluorinated paints. My understanding is that the graffiti literally slides off and people give up after a few tries.

Given the costs, my suggestion would probably be a thick layer of polyurethane. These are often applied to seal wood floors and can be transparent or stained. If you apply multiple layers (say 2-3 on a floor) the surface is very smooth and slick. It's hard for paint or other stains to stick.