How To Stop a Storyboard

I have an application with a storyboard, which turns a icon that indicates that the application is currently busy. Because I don't have any chance to know, how long the application will be busy, the RepeatBehavior on the application is set to Forever and I have to stop it manually.

OK, seams to be easy, the storyboard has everything needed to stop it, a Pause-, Stop- and Remove-method, but somehow, nothing works. After some time I detected one line in the Output-Window of Visual Studio:

System.Windows.Media.Animation Warning: 6 :
Unable to perform action because the specified Storyboard was never applied to
this object for interactive control.; Action='Remove'; ...

After some research the answer was very easy, when you start the storyboard with the Begin-method, there is a second parameter of type Boolean, isControllable. This has to be set to true, afterwards all the methods mentioned above will work.