Hello!
For future questions about skript please use the Skript discussion section of the forums.
In this case you're going to want to use the "on break" event, and compare the block type to ensure its a block you want to allow being broken. If it is not, you can cancel the event to prevent the block from being broken.
If you want this behavior restricted to an area, you can use WorldGuard region compatibility with skript, or SkBee bounds to localize the event.
Hope this helps!