This applies to the Scarlett 18i20, 18i8 and 6i6
To remove Scarlett MixControl from a mac, the following files should be moved to the trash:
Macintosh HD/Applications/Scarlett MixControl
Macintosh HD/Library/Receipts/Focusrite USB 2 Driver.pkg
Macintosh HD/Library/Receipts/Scarlett MixControl.pkg
Macintosh HD/Users/[yourusername]/Library/Application Support/Focusrite*
* indicates that you should delete the entire folder. This folder is located in the User Library, to locate User Library: Go to the Finder (or desktop)>Hold the Option key on your keyboard, and click the Go menu at the top of the screen>With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.