To stop your auto-invest from continuing to invest for you, you simply turn it off. If however, instead, you are wanting to change your auto-invest, please refer to this guide instead: How to: Change my Auto-Invest.
Turning off an Auto-invest
Turning off an auto-invest will stop any future auto-invested from going through. You can then, at any point simply turn it back on and the auto-invest will continue to function on the next auto-invest date.
1. To turn it off - first head over to the Auto-invest page
2. Then simply toggle the ON/OFF toggle button to OFF.