If you are actually trying to develop a package for Composer and you manage to do it without getting yourself into a bloodrage, then you are a much more zen person than me. One of the things that pisses me off the most is that it caches the Packagist index on my local machine, and when I am pushing tweaks to Github several times an hour, that is super annoying.
- Find your AppData\Local\Composer folder. For me, C:\Users\bob\AppData\Local\Composer.
- Delete all the crap in it.
- Right click the empty space you just made, choose Properties. Security tab, Hit Edit.
- Select your user, put a checkmark in the Deny box for Write. Hit Apply and accept its warning that you know you are denying yourself.
- Enjoy Composer being unable to cache.
I assume on Linux you can find wherever it hides it in your home directory and chmod 000 it.