You can uninstall or removes an installed python-pyrex package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove python-pyrex
If you would like to remove python-pyrex and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove python-pyrex
If you use with purge options to python-pyrex package all the configuration and dependent packages will be removed.
$ sudo apt-get purge python-pyrex
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove python-pyrex