Does it have to be full block or can it be anything more to block conduit power? -> like place beetween conduit and prismarine block?
The Wiki says:
"To activate, a conduit needs to be in the center of a 3×3×3 volume of water (source blocks, flowing water, and/or waterlogged blocks)"
So seaweed shouldn't block it, neither would posts and ladders and such as long as they're waterlogged.
Doors would but they'd have to be placed/removed, opening and closing wouldn't affect anything.
If you want to be able to switch it on and off I'd use a block on a sticky piston or find some way to raise and lower the water level.