I'm currently building a mobfarm on a server and looking for blocks where spiders can't climb because they always run up the bay
To my knowledge, they can do this on all blocks but they can't overhang what I do is mostly the top of the opening from the shaft then 2 blocks of space and then the wall, so that the spiders should not be able to touch the wall and if they do hang around in these 2 blocks and do not bother until they push each other down…
Otherwise, if you can do without threads and so you can even in the Mobfarm itself distribute individual blocks, so there are no 2x2 large spawn areas, which does not reduce the effectiveness but there are no more spiders
According to my years of knowledge, they can't raise glass or make a wall of cacti
