I'm not so sure about that...
Screen spanning doctor does not re-flash the Open Firmware, thus doesn't make any permanent changes. It makes changes in the NVRAM, but so do you when you change your Mac's clock time.
I'm pretty sure that starting in Open Firmware and then using following commands will put back the Apple defaults:
reset-nvram
set-defaults
reset-all
After that, the computer should reboot, and screen spanning shouldn't be possible until you re-run the SSD-patch.
I do not have an iMac or iBook to test it, but I really doubt that it's possible to make a permanent change to the Open Firmware without reflashing it. That, in fact, would void your warranty.