Mods are allowed to remove randomly-placed blocks if the player they belong to has been inactive for a month or more, but the world would be a lot more attractive if regular players had this ability themselves. Why not allow experienced (Lvl. 6) players to remove these eyesores?
Obviously griefing is the major concern, but you could put all kinds of limits on the ability (one block per day, costs gold per block, requires some kind of plat tool, etc).