In order to disable Prosper's Automated Quick Invest, navigate your browser to this page on the Prosper website:
On this page, you should see all your saved investment filters. On the left-hand side of the page, you'll see a column titled "Automate".
In order to disable all automated investments, be sure to uncheck all the boxes in this column. After unchecking all boxes, be sure to click "Save Changes".
See the picture below for further details.