Here is what I found (Windows XP):
1. The cache itself is in:
C:\Documents and Settings\<your acct name>
2. You can delete the files from there as you see fit. Though of course it is better if LR is not running when you do so.
3. There is a file called index.dat in this directory. If you make this file read-only, you will effectively disable the caching all together.
4. However you can do better than this. You can add the following two lines to the file: C:\Documents and Settings\<your acct name>\Application Data\Adobe\CameraRaw\Defaults\Preferences.xmp:
You have to place these lines before this one:
Notice the slash direction. The size of the cache is in GB's.
The information has been taken from Adobe Forums.
I hope it will prove useful at least to some of you.