How do I only allow placement and destruction of certain blocks in a specific world?

I've been fiddling with PermissionsEX / ModifyWorld / WorldEdit for ages now, and I can't seem to make it so that in world "arena", you're only allowed to break "logs, leaves, enderchests and saplings", and only allowed to place "saplings and enderchests" (The reason for the enderchests is for the plugin Build-in-a-Box, if you're asking)

My current permissions looks like this, I'm struggling to make modifyworld and PEX work with it negating things. Does PEX have some special syntax when it comes to negating permissions, or something?

Plugins in use: (Although I am willing to use more plugins to fix this issue!)
ArenaCTF, AutoSaveWorld, BattleArena, Build-in-a-Box, ChatManager (unused), CleanroomGenerator, Modifyworld, Multiverse-Core, Multiverse-SignPortals, OtherDrops, PermissionsEX, PlotMe, RecipeManager, WorldEdit, WorldGuard.

EDIT: Further testing has revealed, in PEX debug mode:
"User NeonJ checked for "modifyworld.blocks.destroy.leaves", cache value false found"

 

So at least Modifyworld denied the action.
On the negative nodes: Make sure to put negative nodes at the beginning of each permissions: list, PEX works like first-come-first-server, so if you put positive nodes first the negative ones may be ignored.

 

I think you should delete Modify World. It's really buggy, I deleted it from my server myself. World Guard is much more dependable/flexible. NeonJ