You can create a float (or bool) parameter in the lamp material to control emissive, then create a dynamic material instance in blueprint, apply it to the mesh (if all the lights work at the same time, then one material can be applied to all meshes), and then when you want to turn off the light, set the parameter to 0 on the material instance.
Or you can just create a copy of the lamp material and turn off emissive in it, but this is bad practice in my opinion, as it is unnecessary duplication.
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