How to undo a software installation process in Mac Mini with OSX 10.11
The instructions on the mentioned website led you to install Homebrew and Homebrew Cask, and use it to install qBittorrent. Homebrew is a community developed package manager for macOS.

To completely undo everything that this command did, first uninstall all packages that could have been possibly installed by Homebrew. To do so, follow these steps:


Run brew list to list all the installed packages.
For every listed package p, run brew uninstall p.
Make sure that all Homebrew packages are removed by running brew list and verifying that the command doesn't give any output. Repeat step 2 until all the packages are uninstalled.
Now run brew cask list. This should list all the packages installed via Homebrew Cask. The qBittorrent package should be listed as it was installed by following the instructions in the linked web-site.
To completely remove a Homebrew Cask package, run brew cask zap p where p is the name of the package. For e.g., run brew cask zap qBittorrent to completely remove qBittorrent.
Make sure that all Homebrew Cask packages are removed by running brew cask list and verifying that the command doesn't give any output. Repeat step 5 until all the packages are uninstalled.
Finally, after removing all the packages, you can uninstall Homebrew itself by running the following command-line:


ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)".

Refer official Homebrew FAQ's for more details.