On most servers other people must have your "Trust" to build on or modify your bricks. They do not get this trust automatically, and must request it. At that time you can choose to Allow, Deny, or Ignore the request. Choosing Ignore means any future trust requests sent by them will be automatically denied by the server.
If you want to keep people from building just above your land. You can put a giant modTer cube on it, and modify the modter cube to not render, collide, or raycast. This also lets you build in midair, but whatev.
If the server does not have modTer bricks, you can also use water bricks. But these can be inconvenient. Cause- y'know- water.
Some servers will have a mod which ignores trust entirely, and will allow anyone to build anywhere.
It is not common. But if it happens there is nothing you can do aside from complaining. But like- don't complain. The host probably has that mod installed on purpose.
Some servers will have a preloaded build consisting of Public Bricks. If this is the case you can put a layer of your own bricks on top of the public bricks, or you can use the modTer trick.
So- 3 options.
1. Don't give them Trust.
2. Don't give them Trust, and put some sort of solid shell around your land to prevent them from invading your airspace.
3. Accept that the server you're on doesn't want you to protect your land (if that is indeed the case).