Any trick for deleting locked objects outside boundary?

Just decided to start a scifi themed park and thought the desert textures would make for a great Mars-scape. However, when I started my desert themed sandbox* I found that the map has numerous trees dotted around outside the boundary. These are locked objects.

Is there any way of getting rid of them? I want a barren rocky landscape with the only plants inside the buildings to suit the off-world theme.

* Is a desert themed game always a sandbox game - even when it's in challenge mode [wink]
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