Nexus phones have their super powers when versed against carrier-laden handsets, in the form of software support from Google. The Mountain View-based search giant posts complete "factory images" of Nexus devices for users so that they can flash back to complete stock if they need.
Well, when the Verizon Galaxy Nexus and Sprint Galaxy Nexus (and Nexus) were released, they both arrived in closed-sources bits because of their relation to CDMA and LTE. This stopped Google to release factory images for those three handsets.
Fast-forward to now, and the Sprint Galaxy Nexus now has a complete image available for download that includes everything you'll need to revert back to stock Android 4.1.1 Jelly Bean. Google have stepped up, releasing the Android 4.1.1 Jelly Bean factory images for the Sprint Nexus S 4G and 4.1.2 images for AT&T's Nexus S as well.