On the macOS device:

1. Click Finder.
2. Click Applications.
3. Return to where the Skitch app was previously installed.  Note that the Skitch app has been removed from the device upon un-enrolling.

NOTE: If the Skitch app is still present, it may take a minute or two for the macOS device to receive the un-enrollment command.  Try waiting a minute or two and returning to the list of Applications to confirm the uninstall was successful.