(skript) Disabling Block placing over void (skript)

Hey there. I'm doing a remake of "The Bridges" purely on Skript, but I have one little problem. I've been replicating many features from different plugin versions, but one thing I can't copy comes from the Mineplex version. I want two things:

1. When someone places a block over void, it get's canceled and he gets a warning (I can probably do this, it's not that hard... Maybe some loop with the blocks below not being air.)
2. When someone places a water bucket, and the flow reaches the void, it gets "frozen" (it won't drop down, nor spread anymore)

Do you have any ideas for the second part? I'm completely lost there.