Im sorry but I didn't really understand
1. Did you try to root your phone? And now you want to go back to stock?
2. You managed to root, but it's stock after reboot? Only the superuser app left, but no root rights? And you want it removed? If so, the solution is simple. Menu-->Settings-->Applications-->Manage applications. Scroll down until you find it.
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Samsung Galaxy SIII [GT-I9300]
[4.1.1] Supernexus Latest Build
Samsung Galaxy S [GT-I9000]
Samsung Galaxy S Plus [GT-I9001]
Feacore Phoenix 1.4