What is the phone manufacturer and model? What version of the Android OS is it running?
Since the app is not coming from the Play Store, you most likely have "Allow unknown sources" enabled in Settings; normally Android phones will not allow the installation of apps that come from anywhere other than the play store. I would begin by ensuring that this function is disabled; the way to do so varies by OS version - try this Google search to find the method that works for you:
https://www.google.com/search?q=andr....0.C0fifWWw4Bg
Once you have disabled unknown sources, try uninstalling Pop again and see if it still comes back.
Let us know how you make out.