He means how do you move it as part of the trap. It just isn't possible to have an entrance - from any direction - become blocked by obsidian by a machine. According to my readings notch did this precisely TO prevent this kind of griefing. Obsidian can't be pushed, pushing destroys lava source blocks, and water plus lava flow becomes cobblestone.
The only way to make this work would involve the victim passing through the lava on the way in. You could certainly build a trap that drops them into your lava layer and they might even survive if they think to swim sideways and drop out of the stream of flow and down into a pool of water (to put the fire out) and then a delayed piston releases water over the lava up top to seal it, bu the flow beneath will remain for some time. The trap only works if the player survives entering, and that's a very big if that you can't guarantee.
The only way to make this work would involve the victim passing through the lava on the way in. You could certainly build a trap that drops them into your lava layer and they might even survive if they think to swim sideways and drop out of the stream of flow and down into a pool of water (to put the fire out) and then a delayed piston releases water over the lava up top to seal it, bu the flow beneath will remain for some time. The trap only works if the player survives entering, and that's a very big if that you can't guarantee.
