Hi methusalah,
When you’re using a stationary or static light that has baked in GI will not be disabled since it’s precomputed. One thing you could do is use a Post Process Volume in the area need or set as Unbound and have it use a timeline in a Blueprint so that you can adjust the fade of the GI over time as the light intensity shifts. You don’t have to use a timeline and could simply just set the Global Illumination Indirect Lighting Intensity to 0 via the Post Process Volume settings.
-Tim