I had the same problem till about 10 minutes ago. Giant mailbox outside my lot, basically destroying the view. First I tried the cheat, I set the mailbox as head again, but this resulted in the mailbox being copied and yes I was able to delete that one with the cheat. But not the one outside on my lot. Then I tried the mod, and that worked perfectly! I know it says it hasn't been updated in a while and for those 'scared' of mods, it is safe to use. Just install it, and then open the game, destroy the mailbox, or a certain unicorn, after which you can save and close the game if you like and delete the mod.
In my experience the mod is the only thing that can help you get rid of (any) item outside the boundaries of any lot.
(Go to mod the sims .com, search for 'enable advanced debug' and voila. Good luck!!)
