Please note: I am not looking for a server plugin resolution, I want to know if the question in the title is achievable.
I am playing on a small server with a couple friends, with everyone settling in different areas. We have a claim chunk plugin for factions, however my visitors can still interact with buttons, pressure plates and due to a plugin issue, they can also put down water and lava from buckets.
I have built a safe room in my basement which can only be opened from outside, by placing a block in front of the observer. The redstone circuit is an on/off switch which results in the door opening if a block is present, and the door closing if there is no block in front of it. Or that's at least how it was meant to be used by faction members.
The below is a mock version:
Since the observer does accept lava/water as a block change, players can currently open the safe room by placing water down in front of the observer, due to the aforementioned plugin problem.
Given the space I am limited to (3x3x3), is there a way to prevent water/lava from being placed in front of the observer, without taking away my ability to place blocks in front of it?