It seems Actionmanager.StopCasting(); doesn't do anything? Scenario : I cast cure on myself, execute Actionmanager.StopCasting(); in the Reborn Console it doesn't cancel the cast.
Working fine for me. Your probably doing it too slow if your manually casting it and then pushing the button to stop it. There comes a certain point where the client cannot cancel a spell request.
I use it to cancel overheals and I can confirm it does work fine - keep in mind though that there can be pretty high latency to the server.