It might seem easy to you, but this involves quite a bit of coding actually and more importantly changes to the game itself, which, after release, are kept to a minimum (and I'm not just talking about FC5; any developer avoids making changes to the main game code after release).
So, you would have to add code inside the Editor to be able to set your map to Free / Locked / Read Only, right? Also, add visual support to that (GUI, Button Legend).
You would also have to make changes in the Arcade, to disable the Download button if the map is locked.
Or you could still download it, but then display a message in the Editor, saying it's locked for editing (but then users would be mad because they've wasted bandwith/time downloading it for nothing; so it would be better for the map state to be shown in the Arcade).
The Arcade should also know the state of each map, maybe they'd use an internal flag for this, so the game can tell which map is locked, thus disabling the Download button; and you'd probably want to also show the user the state of the map, so they'd also have to have a graphic symbol for the state on each thumbnail or in the Map Info.
Back to the Editor, if you download a Read Only map, you'd need a message pop-up to inform that you won't be able to save it; also, code support to disable Save/Save As.
Last but not least, you'd need to test these changes.
All in all, that's a lot of time and resources wasted on something that isn't that important (if the 1stCreator bug is fixed, which it should be as of TU5) and that could be used on improving the Editor/Arcade in other ways. Also keep in mind, that, after release, a game's dev teams tend to ramp down.
All I'm saying is, if it wasn't in the game from the start, don't hold your breath that it will be added later.
If it is complicated to give options. Change to not republish by another author, is the option. Who is going to bother? The vague "mappers"?
Those who are annoying are the mappers who invest hours in providing free and original content, either of better or worse quality. But it should not be important to care for and retain these people. Or less important than to please the republicators.