We cannot change it.. It's public. Lots of griefers out there.
But we have LogBlock enabled on that world.
Now we have 4 member areas around the spawn.
Those zones are for members only to make the guests feel a bit
better when they first join the guest world. They will see (hopefully)
nice houses, farms, a market and other stuff you like to build there.
No trusted membership needed. It's free for all members!

By the way:
Please make the market a bit nicer and reduce them to ONE market.
Nasty buildings will be removed.
Destroying in any way results in a server ban.