In addition to pure vandalism where any kind of block is taken just to damage buildings it seems there are also thieves around. They pick specific parts while leaving the rest of the building more or less alone. Most popular items are
* bookshelves
* mese
* signs
* fences
* jungle grass
* ladders
* bricks
* glass
* sand
* torches
I never placed nyan or papyrus in a public place but I guess they would be in the top ten, too. No idea why people steal grass or sand. You can get plenty of it from the map, usually faster than from my buildings. This is a problem on all servers I play, public as well as on restricted.