DirectX 9 HLSL Glow Effect?

You should work with different rendertargets ( Documentation of SetRenderTarget ). First you render all not-selected rings to the backbuffer. Then you draw the selected ring to an extra texture as a rendertarget.

Your glowshader make this texture glowing and finally you render the texture to the backbuffer. So your green ring is glowing and the others aren't effected by the glowshader.

If you want to have glowing lines and don't use the glowshader for anything else you could make your lines glowy with a "thick" line and a appropiate texture as in following picture.

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