If someone in a custom game has a mod that you don't, you'll automatically download that mod. You can limit how much you download with the Custom Aesthetics option, but you cannot stop the downloads completely (unless you avoid custom games altogether).
I did a bit of experimenting and it looks like turning Custom Aesthetics "off" prevents things like decals and skins from downloading. It seems only the required mods (like custom victory strikes and custom maps) will be downloaded.
Since setting Custom Aesthetics to "off" prevents aesthetic mod downloads, there wouldn't be any benefit to changing your decal/skin/etc mod folder permissions. Changing the permissions on the fatalities/gamemode/etc folders would likely cause a sync error if you try to play a custom game without all the required mods.
You could try only joining custom games marked as unmodded (not sure if this accounts for all kinds of mods), but someone with mods could easily join after you do and force a download.
So basically, you can reduce how much you download with the Custom Aesthetics option, or you can stop the downloads completely by not playing custom games.
Thanks, but this switches just the aesthetics off, not the download. COH2 still downloads mods I dont want to have.
I just tried joining a modded game lobby with Custom Aesthetics turned off, and it still downloaded several non-tuning mods.
The only solutions I can think of are to either not join modded lobbies (actually that would only stop the game from downloading a few different kinds of mods, like tuning), or to maybe change the permissions on your mod folder (haven't tried). (see my next post for update)