Is there any block that spiders can't climb ? It's really messing up my farm and I'm looking for a way to prevent it. :D Thanks.
They can climb on any block (except lava, since they die before getting on the walls), but that isn't good idea to prevent them from coming in, as the lava can burn everything. If you use fences, place a carpet on top of them (or a slab, dosen't matter), and if you use blocks, put a trapdoor on them and close it. These blocks stop them from climbing up them, because the blocks the spider climbs on must be flat (e.g. a 3-block stone wall), so carpet on fences and trapdoors on blocks are the best solutions to stop them. The walls don't need to be that big, but since Spiders could try to jump and eventually get on the carpet/trapdoor, ignoring it completely, it's better to make it 3 blocks high.
Hope this helps.