Do you mean the Locomotive Wreck? I found out, that if Nymphs want to harvest something, they choose something nearby. Which means that if you put something they allowed to harvest around the Wreck, they do not harvest the Wreck.
But have at least one spot free, because the steam does not spawn, if the wreck is completly surrouned by other items. (not sure why, since the steam flies, but i think it works like every other item that spawn items over time)