You can find a list of all installed packages by running a command:
composer global info
To remove all global composer packages just:
1. edit COMPOSER_HOME/composer.json
file and remove all records from «require» section
2. run composer global update
to apply the changes
COMPOSER_HOME by default points to:
C:\Users\<user>\AppData\Roaming\Composer
on Windows/Users/<user>/.composer
on macOS/home/<user>/.composer
on other *nix systems