How do you avoid negative RGBA values with matte control?

I'm substracting a polygon with another and the Alpha value seems to go below 0. How do I avoid this? Is there a way to default it to 0 if it goes below 0? How would I go about this?

How are you substracting them? What method are you using?

You don't need MatteControl to subtract a mask. The masks themselves have a paint mode.

They're both right but if you need to floor a channel you can always add a Brightness and Contrast node. There's a check box for Clip Black and White. That will 0.0 black and stop White at 1.0.

Remember to tick the Alpha box if you need to clamp the alpha, though. BrightnessContrast has that unchecked by default.