After clicking "Use Nodes", how do I "unuse" Nodes, other than pressing CTRL+Z?
Select the light, open the shader editor and clear the Use Nodes Checkbox
This won't delete the node tree but will stop using nodes.
You can open the shader editor by going to the dropdown menu in the upper corner of the 3D Viewport, clicking and selecting Shader Editor. (shortcut Shift–F3)
or you can switch to the Shading tab in the top bar menu. The Shading editor will be the bottom editor.