How do I disable Spotlight?
To disable, open Terminal and run

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


This disables the Launch Daemon that starts and keeps it running. You can also run: 

sudo mdutil -a -i off


but from looking online in a few places, this may give you an error like 'Spotlight server is disabled'

If you disable though, it will continue to use an old cache, or not work at all. Therefore, the launching applications as you mention will no longer work. It also may break other functions related to the OS, like the Mac App Store (to know whats installed, etc), and LaunchPad. 

If you still need an application launcher, you can use applications like LaunchBar and Quicksilver.

After seeing your update (espc. on how to stop it indexing so much), there are two things to check:


Are you running something like a virus scanner or other application constantly accessing files on your drive? This will force spotlight to keep indexing. A network share would also be a culprit. 
You can also filter what folders Spotlight should index, and by configuring this, can limit how much has to be indexed. Do do this, go to System Preferences -> Spotlight -> Privacy. 




Re-enabling would be running 

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


or 

sudo mdutil -a -i on