This isn't possible.
Saving bricks is clientsided and anyone can do it.
If you're worried about people stealing their builds and claiming it for themselves, then just place hidden initials or your Blockland ID, or your name in the build.
Even if it's just a random brick, that when clicked pops up with your name.
And really, it's not much of a problem if someone is hosting your build.
I like to think that it's more a compliment, that they think it's good enough to host.
And I'm happy enough to know that other people are enjoying my build.
And if they try to add to it, it's likely that they won't match your style, and it'll be clear who made the original.
Just ignore people who host your stuff elsewhere.
You've still go it on your server, you can still add to it.