You can uninstall or removes an installed elpa-smex package itself from Ubuntu 16.10 (Yakkety Yak) through the terminal,
$ sudo apt-get remove elpa-smex
If you would like to remove elpa-smex and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove elpa-smex
If you use with purge options to elpa-smex package all the configuration and dependent packages will be removed.
$ sudo apt-get purge elpa-smex
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 elpa-smex