Are there any blocks that spiders cannot climb?

I'm building a spider spawner exp farm, and I have everything planned out.

Now the only thing is to build it. But what material should I use?

Are there any blocks that a spider (either type) just cannot hold on to the sides of?

My farm should function if there are none, but it would be a huge boost to convenience and efficiency if there are any such blocks.

Keep in mind that this question is not the same as this question which just asks how to make your house safe from spiders.

Solution 1:

Cover the walls with signs. Spiders can't climb a wall if it's covered in signs. I just tested it.

Spiders can climb:

Any solid block, that is, one that obstructs the players movement. This includes 'solid' blocks such as stone, wood, dirt etc. as well as 'transparent' blocks such as fence, glass panes and iron bars.
Ladders and vines, exactly as the player can.

Spiders cannot climb:

Blocks which do not impede the player, such as grass, sugar cane, fire or flowers.
Water or lava, but will behave as other mobs (swim/drown, burn).
Nether portal blocks (they can climb the surrounding obsidian though).

They can climb any solid block. If you want to stop them from escaping, make an overhang inside the trap like so:

XX
X
X
X

X=solid block.