In my experience this was never a huge problem -- if I remember correctly, servers I played on actually had some ability to "own" areas of the server so you were the only one with edit privileges for that area. And of course there's always the idea of hiding your construction underground or underwater somewhere hard to find :)
From what I understand edit privileges were done using server admin/moderation tools, not sure how well that scales to big mmo type world with thousands of players, especially when you want people to collaborate on big builds.
