Completely Remove Slack App from Mac OSX

Once you have removed the Slack App from the app drawer manually, some slack files will remain. (this may still be true even if you used a third party uninstaller application)

Now, these files arent in the common/usual Mac OSX locations eg ~/Library/Application Support/ etc

Instead they are located at:

~/Library/Containers/com.tinyspeck.slackmacgap/ in the form of a folder.

Using the terminal you can confirm the presence as such:

ls -lrt ~/Library/Containers/com.tinyspeck.slackmacgap/Data/

which will give you a long-form folder listing. Once presence is confirmed you can remove the lot by:

rm -rf ~/Library/Containers/com.tinyspeck.slackmacgap/