To my suprise there is no such setting in my stock Nexus S (4.0.3). Thanks to the comunity after few googling around i found there is this hidden setting that allow your phone to select to use WCDMA network only as currently it is been set as WCDMA (preferred)
Dial *#*#4636#*#* and It will bring you to Testing Screen, for ICS you can click on Phone Setting and change the network to WCDMA only to fix your phone to use only 3G network.
Not able to make Screen Shot with the full text ( *#*#4636#*#* ) as it will auto go to the Testing Screen when you hit the last input.
The setting available to for the network selection.