If your friends are alright with playing without spiders, the admin of the server can give themselves a command block (
/give @s command_block), place it (preferably somewhere normally out-of-reach), and enter the command
tp @e[type=minecraft:spider] 0 -80 0 into it, setting the options to Repeat, Always Active, and Unconditional.
This will immediately teleport any spiders that happen to spawn under the world so they can fall to their deaths.
Alternatively, if you want free string and/or spider eye items, you can kill them where they stand with
If you don't want to bother your friends too much, you could instead only murder spiders that happen to be around you. To do this, add the tag
arachnophobia to yourself with
/tag <your_name> add arachnophobia and then put this command in the command block:
/execute as @a[tag=arachnophobia] at @s run tp @e[type=minecraft:spider,distance=..32] 0 -80 0. You can change the distance (32 is used here) as required.