Add a wall sign and disable "monster"

[PureTryOut]

And my other question is how to disable the spawning monster?

HOOK_SPAWNING_MONSTER and cMonster, docs are handy
First add the hook for HOOK_SPAWNING_MONSTER, and in there check what type the mob is (I'm assuming you want to disable hostile mobs) using cMonster:GetMobFamily(). Then if you want to disable the mob from spawning, return true. Otherwise, return false and the mob will spawn.

You can find an example here.