The grafitti should be removed as soon as possible. Otherwise, when other people encounter the grafitti, some may be encouraged to add their own grafitti as well. This is similar to the "broken window" syndrome - a broken window that remains in a building encourages other windows to be broken, but if that window is fixed promptly, then it is less likely that other windows will be broken.
For example, at one time New York City train and subway cars were noted for being covered completely with grafitti, because the presence of grafitti on train cars encouraged others to add their own, and vandals who painted the grafitti could look back and see the results of their destructive actions for a long time. When the authorities decided to clean up the cars and made a concerted effort to remove grafitti as soon as it was applied, the occurrences of new grafitti declined. Vandals saw that their "works of art" disappeared soon after they applied them, and thus they became discouraged and consequently less likely to add new grafitti. This discouraged other vandals as well. Therefore, subway cars remained clean and free of grafitti.
People should be on the lookout for vandals who commit such destructive acts, not only in Wilderness Areas but anywhere on public and private property. These people should be arrested, convicted, and given harsh punishment.
Wasn't there also a crackdown on security in subway yards or car barns? This might discourage "taggers"; apparently the hazards of hot third rails at night and possible encounters with hostile transit police weren't enough.
As I made my way to WI this summer, I got a call that the HP of Kansas had been vandalized. I stopped there and found out that only the mailbox has been damaged, but noted that several people had written their names in pencil on the wooden fence posts surrounding the HP sunflower, etc. I used a combination of pencil eraser, sand (grit) and water to rub the writing off the posts, without leaving an obvious erasure mark. It seems to have worked. Haven't been back, but hopefully the evidence of the graffiti is no longer evident. BTW, one set was from Colorado, the other from Scotland! Getting the stuff off, even if it requires natural materials and elbow grease, will help discourage others. Of course, prehistory and old historical stuff requires additional consideration. See ya! JES