That's not really the idea - ancient graffiti is of vastly more value that the mere fact that someone wrote it. Ancient graffiti gives us insight into language

That's not really the idea - ancient graffiti is of vastly more value that the mere fact that someone wrote it. Ancient graffiti gives us insight into language and pronunciation (misspelling is very useful in reconstructing pronunciation), aspects of life considered taboo or otherwise unworthy of educated record (sex, usually), the lives of the comparatively poor and unimportant. It tells us about who was where, when, and doing what, in a way official records (if they exist) often don't. Many of the things we've learned from graffiti can't be learned elsewhere. Things really would be lost to future historians if we managed to stamp out modern graffiti. Thankfully, we won't, so the question is moot.