Diagonal tunnels downwards won't work, you will still have to go into the lava to place anything.
You start by making a 6m tall wall in the outer edge of the flow wall. The lava will spill over, so you add a row out toward you and up one meter, to keep the flow from coming toward you. Then you mine out the bottom 5m of the wall, and build a new wall in the lava flow one meter inward. Then you dig the bottom 4m out and move in again. You end up with a diagonally-slanting roof piercing the lava flow.
You just have to start with a wall as high as the lava flow is deep.
Using water won't work either, the lava is vertical, a giant cube of lava with your house in the middle.
You pour the water, it hits the lava, and turns the lava flow into cobble. Then you build on the side and then on top of the side block to make a rim so the lava flow lands on the cooled cobble and stops there. Mine out a little tunnel and repeat with water on newly exposed lava.
What stops someone from hopping to the sky limit on a column of sand and building cobble down, out, and up to fill in the lava source blocks while preventing backflow onto the sand column?
Finally, as a side note, lava is not infinite. Where are we going to find 30x30=900 lava blocks to shroud the house with? And again, I'm not seeing how the house can be both lava-sealed and also accessible with a combo lock, such that the combo lock linking mechanism is impervious to someone tunneling through it.
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I really think the anti-griefer measure is fake honeypots. You have a fake outer house, a fake inner house, and a "secret room" with combo lock that seems really cool but he can get through. In the next hillside over you have your real storehouse underground covered in sand.
Then again, anyone can use a texture pack that lets them see through typical ground tiles. So nothing is secret!