The Add Comment link can be hidden by editing the ProcessInfo webpart that contains it. Go to the page list and find the process view page you want to edit. Click the "Go to Page" button. At the top of the screen find the "Site Actions" button. Click Modify Page. Then (i think) open Site Actions again and click Edit Page. This will make a lightning bolt icon appear on all the webparts. Click the icon for webpart you need to edit and open the webpart editor. You'll find (kind of towards the top) a setting called "Add Process Comment". It's a dropdown of display options. Select "None". Save the webpart config. Good to go.