You can uninstall or removes an installed jflex package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove jflex
If you would like to remove jflex and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove jflex
If you use with purge options to jflex package all the configuration and dependent packages will be removed.
$ sudo apt-get purge jflex
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 jflex