You can choose one of the following commands in the Track > Track Automation menu (or the corresponding key command) to delete track automation data:
You can also delete multiple nodes by doing one of the following:
Shift-Option-double-click anywhere on an automation track to open a dialog that allows all automation data (of the current type) to be deleted. This is as per the Delete All Automation Data of Current Track command.
Press Delete to delete a selection of automation nodes. Make sure that no regions are selected when using this function. Any selected regions are also deleted.
Click it with the Pencil tool or Eraser tool.