Prague is covered in tags which is a form a visual smog. They serve no creative purpose  and clutter the city in destructive nonsense. In an anonymous survey made in New Zealand  found that 63% of graffiti writers were under the age of 21. So if we assume its even remotely similar in Czech Republic a very simple solution would be selling sprays and other permanent inks as a controlled product. Only selling these products to individuals over the age of 21. This would decrease the accessibility and thus there would be less incentive to commit acts of vandalism.

Just for a rough comparison this how it would look with the new law instated. Decreasing accessibility would possibly push  vandals to more productive thought provoking acts of vandalism that could add an interesting flair to a otherwise scribbled upon city.