Seems hard to believe that a root and wipe (combined) wouldn't get it out.
If you're planning to go to Sprint and want a Nexus S and push comes to shove you can't fix that one software wise - I do have a Sprint model sitting around unused (although I'll have to figure out where the wife hid it, was on her desk before some spring cleaning... and not seeing it there this second) but in need of a screen replacement that I'd be more than willing to let go for whatever it costs to ship it. [Also a Galaxy Nexus in the same boat - cracked screen (that works) and only would ask shipping]
However as a warning for using it on Sprint, Sprint is going to start getting rid of the 4G band the Nexus S uses in 2015 sometime - so if that's a dealbreaker you might want to save the hassle and get something newer. (Personally I can't imagine using it anymore after our recent upgrade - note the GNexus does use LTE so will not lose 4G when they turn that band off)