To remove a globally installed package run:
composer global remove phpunit/phpunit
global command lets you to run many commands like install, require or update as if you were running them from the COMPOSER_HOME directory.
Read the related documentation here: http://getcomposer.org/doc/03-cli.md#global
COMPOSER_HOME depends on your system (on Linux it’s ~/.composer), see http://getcomposer.org/doc/03-cli.md#composer-home for more details.