Thanks! 
Here’s a version of the plugin that will hopefully largely avoid the issue. Unzip the file to your Desktop, then move the Marcus-InteractiveLight.zsc to your ZStartup/ZPlugs. Restart ZBrush and you should find a new button in the ZPlugin>Misc Utilities sub-palette called Marcus InteractiveLight.
You need to assign a hotkey to the button as it won’t work without. You also need to have the cursor over the canvas area when you press the hotkey (so perhaps choose something that you can press with one hand).
Press the hotkey and then move the cursor around to position the light. If you move off the canvas area the plugin will stop. You can also click the mouse/pen to stop. If you get the blue donut or ZBrush does its busy thing, just wait as it will soon stop. This version has a short loop which will exit quite quickly. Because of this, you may need to press the hotkey again if the plugin stops before you have done adjusting the light.
Download here:
HTH,