disable-spotlight.md

Disable:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Enable:

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

spotlight-off.md

Commands

Disable the index:

mdutil -i off /

Enable the index:

mdutil -i on /

Erase the current Spotlight index:

mdutil -E /

Help

Usage: mdutil -pEsa -i (on|off) -d volume ...
	Utility to manage Spotlight indexes.
	-p             Publish metadata.
	-i (on|off)    Turn indexing on or off.
	-d             Disable Spotlight activity for volume (re-enable using -i on).
	-E             Erase and rebuild index.
	-s             Print indexing status.
	-a             Apply command to all volumes.
	-V vol         Apply command to all stores on the specified volume.
	-v             Display verbose information.
NOTE: Run as owner for network homes, otherwise run as root.

Sources:

• http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/mdutil.1.html
• http://www.mikesel.info/disable-spotlight/
• http://osxdaily.com/2007/03/22/how-to-completely-disable-spotlight/

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
Unload failed: 5: Input/output error
Try running launchctl bootout as root for richer errors.

sudo mdutil -a -i off is better than sudo mdutil -i off / as it will disable all indexing.
However, you didn't mention that it does so TEMPORARILY.