Select the channel >Open channel menu > the first option is "Automatisation",a second menu appears,and there you choose " Cancel all visible automatisation for selected track.
In the Arrange page select your track;
Now go to the <Track> Menu item and choose <Track Automation>
|the 7th item from the top named <Delete All Automation> is what you're looking for.|
Let as no if you have managed to find it.