I don't think there currently is an option to allow this, but it seems it would be very easy to add this.

As Facepunch deliberatly placed a no build around all monuments and  caves (several years ago) to stop players building on them i dont think it would be that easy, the whole point of it was to stop clans from "claiming" monuments,  afaik there is a no build on roads policy introduced a little while ago too, so much so that the plugin stopping players from building on them was of no use anymore.

Duplicate thread that's already been answered :p