I think you would have to follow the initial directions listed in the "How to Enable Sideloading" sticky thread and instead of copying files you would be removing them. Something like this:
DISCLAIMER: I HAVE NOT TRIED THIS AND CANNOT GUARANTEE IT WILL WORK AND WILL NOT BRICK YOUR PHONE (but I'm pretty sure it won't)
I borrowed the first and last parts of the directions from the sideload thread.
You'll need the Android SDK (
guide to download and install is here)
You'll need the USB drivers (Stickied post in this forum)
You'll need root access (look for the Captivate Root thread by chaoscentral)
You'll need Busybox (available on the market after you root the phone)
Connect USB to phone with USB Debugging ON
Command prompt
cd to SDK install directory
cd tools
adb shell
su
rm /system/app/Superuser.apk
rm /system/xbin/busybox
rm /system/xbin/su
exit
exit
Then reboot the phone. I
THINK that should do it. It should at least remove the superuser ninja icon and not make it obvious that it's been rooted. Hopefully there's an easier way to do it but in my initial googling yesterday I didn't see anything very helpful.