Ge jem31 microwave will not shut off when door is open

This is very dangerous, unfortunately it is also not easy to fix. There is a small switch behind the plastic actuator that is malfunctioning. You will need to remove the shell. You may need special tools (torx, security) to remove the screws at the back around the perimeter. Be certain you have removed power before you do any work. Also be careful when you remove the shell as the edges will be sharp. Hopefully once you remove the shell you can see the switch (look down from above) and determine if you have the skills and tools to perform the tear-down necessary to get to it. It will not be easy to find a replacement unless you have a good parts store in your area. In my experience this kind of failure requires an almost complete tear-down and it may be easier to leave to a local appliance repair shop who will have the parts and experience to not make a pig's breakfast of it.