What material does not reflect infrared light?
A possible readily available material with a low IR reflectivity could be some sort of black fabric. You can try to compare IR reflectivity of different materials by laying them side by side in front of the camera, in a dark room, and checking which one looks darker.
Still, for the camera to "see" through the window at night, you'll need openings for the camera and for the IR LED(s), so the reflections from the glass cannot be completely avoided.
To minimize IR reflections getting back to the camera, you can place the camera as close to the window as possible or turn the internal IR illumination off and provide a separate IR source. These techniques are described in more detail in this article.