You can decrease the level of a water or powder snow cauldron, through the use of a flame bow in spawn protection.
Steps to Reproduce:
- Start a server and join it.
- Give operator permissions to yourself.
- Set the world spawn to your current location.
- Place some powder snow or water inside of a cauldron.
- Obtain a flame bow.
- Remove your operator permissions.
- Give operator permissions to any person other than yourself.
- Shoot an arrow using the flame bow into the water or powder snow cauldron.
→ Notice how the level of the water or powder snow cauldron is decreased, despite it being in spawn protection.