Go into the menus for your phone. Most likely something like:
Menu, Settings, Networks, Mobile Networks, System Select. Then select HOME ONLY. This will force the phone to stay on the Page Plus network. However, in an emergency if you are in a roaming area and need to place a call you will need to go back and select either Automatic A or Automatic B.
For more detailed information search for your telephones' user guide online for help.