Can I put stickers on other players' buildings? How do I remove one from mine?

Yes. Stickers are the one thing you can place inside someone else's building zone — you don't need Tool Cupboard authorization, and the placement preview stays green.

Removing a sticker. Only the player who placed a sticker can pick it up. Anyone can shoot it off — a sticker is a separate small object with very little health, so if someone stuck one on your wall, just hit or shoot the sticker; you don't need to damage the wall, and the wall isn't involved. A hammer won't remove it — decorations can't be demolished that way.

Worth knowing. A sticker on your wall costs you nothing (no upkeep impact). You can repair a sticker on your own building for metal fragments if you want to keep it. On PvE servers the usual damage rules apply, but as the owner of the building you can always remove a sticker from it.