I was looking around on google for a way to make blocks unplaceable and I coded my own with Skript. I thought I would be helpful and just publish it for any new Skript users needing it and who couldn't code it!
on rightclick with {original name of the block/item you want unplaceable}:
name of player's tool is "{name of unplaceable item}"
cancel event
on rightclick with {original name of the block/item you want unplaceable}:
name of player's tool is "{name of unplaceable item}"
cancel event