Also please read more carefully, before going "Awful suggestion" with no feedback.
They can still add/delete blocks to their OWN structures.
I've outlined my opinion on this numerous times, I provided a link to where it was outlined along with the opinion of many others, however if you want me to repeat it here; sure. The main reason minecraft is awesome is because of the freedom we have, if we want to build a swimming pool, a castle, a giant spaceship or a giant phallus, we can! What you're suggesting is that you remove this freedom and limit new players in what they can do. If when I first joined minecraft I'd only been able to delete my own blocks I'd have quit instantly, this isn't gmod.