First, you want the areas of the "mask" that you want to not glow be black, and then connect it through a Math Functions node to the Ambient Color.

In the attached picture, the black areas in the second image map will render whatever the Color Texture and Diffuse Color are, while only the white areas will render with the Ambient Color.

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.