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