Reducing the sensitivity of a PIR sensor?
You can attenuate by using thin sheets of polyethylene. Polyethylene is the same material that the PIR lens is made of. It will pass, but attenuate the wavelengths you are interested in. Find something around .015" and begin stacking until you hit your desired range.

Here is a source: http://www.mcmaster.com/#polyethylene-plastic-sheets/=hc4uvj