I'm not shure what cache you mean. 

ARM L2 cache: 128kiB; probably not a good idea to disable it, the pi might become awfully slow. From http://elinux.org/RPiconfig : setting
`disable_l2cache` in config.txt disables ARM access to GPU's L2 cache. Default 0.  
Needs corresponding L2 disabled kernel. 
This means: compile a custom kernel, replace your kernel.img etc with 
your custom build on the vfat partition of the SD card, and keep a copy of the files somewhere. Or grab & install berryboot (berryboot.com).
 
Linux kernel cache: from  http://www.commandlinefu.com/commands/view/2888/release-memory-used-by-the-linux-kernel-on-caches :
> free && sync && echo 3 > /proc/sys/vm/drop_caches && free


On 2nd thought, I assume you mean locking cache **out**, aren't you?
> echo 10 > /proc/sys/vm/swappiness

will swap as little as possible; default is 60. Note that I didn't try, I just removed the swapfile.