Uninstall slack
To uninstall just slack package itself from Ubuntu execute on terminal:
sudo apt-get remove slack
Uninstall slack and it’s dependent packages
To uninstall the slack package and any other dependant package which are no longer needed on Ubuntu.
sudo apt-get autoremove slack
Purging slack
If you also want to delete configuration and/or data files of slack from Ubuntu then this will work:
sudo apt-get purge slack
To delete configuration and/or data files of slack and it’s dependencies from Ubuntu then execute:
sudo apt-get autoremove --purge slack