Unfortunately this will always be an issue... not all people of earth have respect for our planet and history. I'd even go so far as to say that many of those that don't, also don't have an IQ high enough to even understand, and so they don't care.
I was in Slovakia last year and one of the oldest walls in the world, the remnants of a castle wall was completely destroyed by graffiti. This picture was the cleanest spot I could find.
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