On Minecraft servers, how do they make it so that there's no land outside of maps?

So on Minecraft I have glitched my way to get outside of the maps on many various servers. When I get outside the map, I notice that on the maps there's only a little bit of land outside the map but everything else is just a hole that goes to the void. I understand that they can just destroy the land to do this, but my question is HOW they destroy all of that land so easily.