In the 3d viewport I am using a border region to only render things inside the selected region. I did this with CtrlB. I cannot get rid of the region. How can I make the border disappear?
Use Ctrl + Alt + B to clear the render border, or if you open the Space menu and type "render border", you should see the option.
There is also Shift + B, but this is the legacy shortcut for border render IIRC and it only works when looking through the camera. Use Ctrl + B as you can have different render borders for the viewport (when doing realtime rendering) and the camera.
On the View Properties Panel (N to open it) you can find the "Render Border" checkbox. So, select/unselect the chebkbox to enable/disable the render border appearance.