Minecraft isnt caching it in the traditional sense of the word, the data has to be stored in memory owned by minecraft at some point, in order for minecraft to do anything with the data. Even reading it!
Therefor it is not possible to totally disable 'caching'. Its also not possible for minecraft to just use the memory of the RAM disk directly, it has to go through the operating system first, which adds quite a bit of latency (nothing compared to HD access, but compared to RAM access).
So it's not useful to just disable caching, it might be possible to reduce the time until chunks that aren't near any players are unloaded with a plugin, but I have been unable to find one.
One hackish way that might work is to reduce the maximum memory minecraft can use, that would force it to unload unused chunks faster.
Of course, the only wasted RAM is RAM that's not being used, so (on a dedicated machine) there is no harm in letting minecraft store something it can access very fast.