It can be removed via CSS.
The only way that I know of, would be to use a property change event on the pens prop, and effect the changes after they have been added.
Currently there no way (that I am aware of) to hi-jack the actual js that is executed when the "add pen to chart" button is clicked.
You can of course provide your own tag browse component, and then script the addition of pens the chart.
I would say this is probably a bug. Contacting support is the official way to report bugs, and find out if IA is aware of them or not.