You can uninstall or removes an installed pythia8-root-interface package itself from Ubuntu 14.04 LTS (Trusty Tahr) through the terminal,
$ sudo apt-get remove pythia8-root-interface
If you would like to remove pythia8-root-interface and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove pythia8-root-interface
If you use with purge options to pythia8-root-interface package all the configuration and dependent packages will be removed.
$ sudo apt-get purge pythia8-root-interface
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 pythia8-root-interface