How can I stop it and keep it stopped until I need it?
In my opinion, that is a poorly coded app. Some apps need to stay running, which I agree with. Apps like the launcher, cell services, etc., need to stay running, and sometimes will go dormant when not in the foreground. The Sam's app, for whatever reason, is setup to start upon destroy.
You can see if maybe there is a permission in Settings>Apps, but I don't think you are going to have much luck.
I would email the developer and ask them why. Depending on what the app does, you can uninstall it, create a bookmark on your browser, and have the shortcut to that bookmark on your homescreen (which really all a mobile application is just a bookmark and highly customized webpage).
Edit: I found a permission on the Play store that is called run at startup. That might be the one to deny.