I'm not aware of any way to stop a play ringtone action. The problem had to do with underlying android and the need to reference the original play ringtone instance.
Most people use music play instead with a music stop action.
The cancel alarm action is for system alarms, which you can set through tasker, but you would need to use an alarm set action.
When you choose a stream in a audio action (such as the alarm in your play ringtone), you're determining which volume control affects the sound. It's not the same as creating a system alarm.